KONSTANTIN KHRAMTCOV

ANALYSIS OF POWER SUPPLY METHODS FOR WIRELESS

BIOMEDICAL SENSORS AND FUTURE DEVELOPMENT PROSPECTS

Master of Science Thesis

Examiners: Prof. Leena Ukkonen

Prof. Lauri Sydänheimo

Examiner and topic approved by the

Faculty Council of the Faculty of

Electrical Engineering

on 30th of August 2017

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ABSTRACT

KONSTANTIN KHRAMTCOV: TUT Thesis Template

Tampere University of Technology

Master of Science Thesis, 68 pages,

June 2018

Master’s Degree Programme in Electrical Engineering

Major: Wireless communications

Examiners: Professor Leena Ukkonen, Professor Lauri Sydänheimo

Keywords: Implantable biomedical devices, Wireless powering methods, Implants,

Energy harvesting, power management, inductive coupling, modulation techniques

Implantable biomedical devices have been designed to provide monitoring and diagnosis of the

physical information in human bodies. The population is ageing and there are many increasing

chronic diseases at all ages all around the world, that’s why modern methods of diagnosis and

monitoring chronic diseases play a vital role in medicine and human health. Biomedical wearable

devices and implantable devices are in high demand due to their wider applications. Wireless

powering methods can improve the way a patient’s health can be monitored and also help patients

who are living far from the hospitals.

The current thesis has been focused on the analysis of wireless powering methods for implantable

biomedical sensors, their advantages and drawbacks, requirements of different approaches as well

as current and future challenges. In order to implement a wireless powering method, developers

have to establish quality management systems in the designing phase, to implement safe and

effective devices, and meet the requirements of regulatory authorities. These wireless powering

methods and devices can improve quality of life to the patients and extend their lives.

However, one of the significant challenges is the power supply, because it is vital to provide

sufficient power and to maintain it on the same level along the whole time when the system is

operational. Modern wireless IMDs require a stable and continuous power consumption; that’s

why this problem becomes significant.

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PREFACE

This thesis was made as a part of the requirement for completing the Master’s in Wireless

Communications.

I express my deep gratitude to my supervisor, Professor Leena Ukkonen, who patiently supervised

the thesis and her support, comments and guidance has led the work to successful conclusion.

Indeed, without her support, this work would not have been possible. It was a long and very

exciting work: I really enjoyed this period. I started my thesis in August 2017. Later in May, I was

offered an internship from a London based start-up company, I was working there for one year

and completed it in June 2018. Within this period, I was working and writing the thesis at the same

time.

This thesis was performed at the Department of Electronics and Communication Engineering in

the Tampere University of Technology, Finland. First of all, I decided to divide my thesis into

several parts, at the initial stage I tried to identify modern publications and articles from well-

respected journals. At the second stage, I compiled all my observations and notes together, and

wrote the main part of the thesis. At the third stage, I learned more specific topics such as

cybersecurity for IMDs and types of antennas which employ IMDs.

I also want to thank my colleagues and friends at the Tampere University of Technology for their

valuable comments, suggestions, encouragement, and support through the process. Special thanks

to my parents for their support at every stage of my life.

Tampere, June 09.05.2018

Konstantin Khramtcov

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TABLE OF CONTENTS

1. INTRODUCTION ...................................................................................................................1

1.1 Research Objective.................................................................................................................2

1.2 Thesis Outline ........................................................................................................................2

2. SENSOR TECHNOLOGIES ..................................................................................................3

2.1 History of implantable biomedical sensors ...........................................................................3

2.2 Sensor characteristics ............................................................................................................5

2.2.1 Sensitivity .......................................................................................................................5

2.2.2 Measurement range.........................................................................................................6

2.2.3 Precision .........................................................................................................................6

2.2.4 Accuracy .........................................................................................................................6

2.2.5 Linearity .........................................................................................................................6

2.2.6 Hysteresis .......................................................................................................................7

2.2.7 Resolution .......................................................................................................................7

2.2.8 Response Time ...............................................................................................................7

2.3 Frequency bands for wireless bio-medical implants .............................................................7

2.4 Security of wireless medical devices ...................................................................................10

3. MODULATION TECHNIQUES FOR IMDs .......................................................................12

3.1 Principle of ASK modulation technique ..............................................................................13

3.1.1 ASK modulation method for implantable biomedical sensors .....................................14

3.2 Principle of PSK modulation technique ..............................................................................15

3.2.1 PSK modulation method for implantable biomedical sensors .....................................15

3.3 Principle of FSK modulation technique ..............................................................................16

3.3.1 FSK modulation method for implantable biomedical sensors .....................................17

4. POWERING METHODS FOR IMPLANTABLE BIOMEDICAL SENSORS ...................19

4.1 Lithium batteries ..................................................................................................................21

4.2 Nuclear batteries ..................................................................................................................21

4.3 Piezoelectricity power generators ........................................................................................22

4.4 Thermoelectricity.................................................................................................................26

4.5 Electrostatic generators........................................................................................................29

4.6 Electromagnetic harvesters ..................................................................................................32

4.7 Ultrasonic energy transmission ...........................................................................................34

4.8 Photovoltaic infrared power radiation .................................................................................36

4.9 Inductive coupling ...............................................................................................................37

4.10 Far Field Communications ................................................................................................40

4.11 Mid-Field Communication ................................................................................................40

4.12 Bio-fuel cells .....................................................................................................................41

5. CURRENT CHALLENGES AND FUTURE PROSPECTIVE OF IMDs ...........................43

5.1 Power management..............................................................................................................43

5.2 Biocompatibility ..................................................................................................................44

5.3 Health issues related to wireless power transfer energy ......................................................45

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5.4 Size of the IMDs ..................................................................................................................46

5.5 Frequency band selection ....................................................................................................46

5.6 Antennas ..............................................................................................................................47

5.6.1 Dual-band operation antenna ........................................................................................47

5.6.2 Implantable antennas for wireless power transfer ........................................................48

5.6.3 Integrated implantable antennas ...................................................................................48

5.7 Security ................................................................................................................................48

5.8 Amplifiers for IMDs ............................................................................................................50

5.9 Future prospects ...................................................................................................................51

6. CONCLUSIONS ...................................................................................................................53

BIBLIOGRAPHY .........................................................................................................................54

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LIST OF FIGURES

Figure 1. Examples of biomedical devices. [160] ..........................................................................1

Figure 2. Cardiac before and after the wireless transfer ...............................................................4

Figure 3. Timeline evolution of IMDs .............................................................................................4

Figure 4. Sensors sensitivity [161] .................................................................................................5

Figure 5. Linearity representation [161] ........................................................................................6

Figure 6. Hysteresis curve [161] ....................................................................................................7

Figure 7. The architecture of MICS system ....................................................................................9

Figure 8. Modulation techniques used in biomedical sensors ......................................................12

Figure 9. The architecture of the wireless battery-less interface .................................................13

Figure 10. The principle of ASK modulation [209] ......................................................................13

Figure 11. PSK modulation waveforms. [162] .............................................................................15

Figure 12. FSK modulation principle. [163] ................................................................................17

Figure 13. Human’s power resources ...........................................................................................19

Figure 14. Energy harvesting methods used in the biomedical sensors .......................................20

Figure 15. The efficiency of different powering methods for IMDs ..............................................20

Figure 16. Nuclear pacemaker [164] ...........................................................................................22

Figure 17. Energy conversion scheme ..........................................................................................22

Figure 18. The Basic work principle of piezoelectric IMD ..........................................................23

Figure 19. Configurations of piezoelectric cantilevers [170] ......................................................24

Figure 20. Configuration of cymbal piezoelectric device [170] ...................................................24

Figure 21. A prototype of shoe mounted PEH [44] ......................................................................25

Figure 22. Dependence of the efficiency of the linear branch cross-section dimensions [158] ...27

Figure 23. The architecture of thermoelectric generator .............................................................28

Figure 24. Conceptual view of the electrostatic generator ..........................................................29

Figure 25. Energy conversion principles of electret-free electrostatic IMDs. [208] ...................30

Figure 26. Electret-based electrostatics conversion model. .........................................................31

Figure 27. Capacitor structure .....................................................................................................31

Figure 28. Types of mechanical generators: a) relative movement, b) rigid body .......................33

Figure 29. Schematic of ultrasonic IMD. [165] ...........................................................................35

Figure 30. Infrared power radiation system [202] .......................................................................36

Figure 31. The architecture of Inductive coupling powering method ..........................................37

Figure 32. Types of topologies ......................................................................................................39

Figure 33. The design of bio-fuel IMD .........................................................................................41

Figure 34. Types of body wireless communication .......................................................................47

Figure 35. Overview of the future system .....................................................................................51

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LIST OF TABLES

Table 1. Table of comparisons between different wireless method .................................................8

Table 2. Examples of comparisons of modulation techniques for IMDs.......................................18

Table 3. Power consumption rates for different IMDs ..................................................................28

Table 4. Comparison of Electromagnetic, Electrostatic and Piezoelectric powering methods. ...33

Table 5. Comparisons between different electromagnetic power methods ...................................41

Table 6. IEEE C95.1-1991 Maximum electric and magnetic field strength limits for public use 45

Table 7. Current trends addressing various threats to IMD security ...........................................50

Table 8. Comparisons between different power approaches ........................................................50

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LIST OF ABBREVIATIONS

ANSI American National Standard Institute

ARQ Automatic Repeat Request

ASK Amplitude shift keying

BFC Bio-Fuel Cells

BPSK Binary Phase Shift Keying

DPSK Differential Phase Shift Keying

FCC Federal Communication Commission

FDA Food and Drug Administration

EBC Enzymatic Bio-Fuel Cells

ECG Electrocardiogram

EMG Eelectromyography

EOG Electrooculography

ERG Electroretinogram

FEC Forward Error Correction

FSK Frequency – Shift- Keying

IMD Implantable Medical Device

IoT Internet of Things

IMD Implantable Medical Device

ISM Industrial, Scientific and Medical

MFC Microbial Fuel Cells

MICS Medical Implant Communication System

NFC Near Field Communication

PVs Physiological signals

PSK Phase Shift Keying

PTE Power Transfer Efficiency

PEH Piezoelectric Harvester

QoL Quality of Life

RF Radio Frequency

RFID Radio Frequency Identification Technology

RX Receiver

SAR Specific Absorption Rate

SIR Signal to Interference Ratio

TEG Thermal energy generation

TX Transmitter

WEP Wired Equivalency Privacy

WMTS Web Map Tile Service

WPA Wi-Fi Protected Access

WPT Wireless Power Transfer

WPAN Wireless Personal Access Network

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1. INTRODUCTION

Recently, the great variety of implantable medical devices (IMDs), have been designed for

different medical purposes. These devices help to provide better treatment for patients for whom

traditional and conservative treatment approaches have failed resulting in significant efforts being

devoted to it. Bio-medical implantable devices were invented more than six decades ago. The first

IMD was designed by E. Bakken; he developed the first cardiac pacemaker.

What is an implantable biomedical sensor? This is a device which is able to monitor and process

information inside the human body; for instance, neurostimulators, implantable cardiac

defibrillators, implantable drug delivery systems, pacemakers, insulin pumps and other devices.

Figure 1. Examples of biomedical devices. [160]

The development of implanted sensors is significant, due to their use in diagnostics and monitoring

health of patients. These sensors are able to continuously collect vital information from a person’s

body, such as force, temperature, and pressure inside the human body and provide it to their

doctors or healthcare centres. Moreover, on-body monitoring can alert the patient of any health

hazard and hence to promote rapid corrective clinical action if the patient is out of the clinic. Since

the 1950s, when the first device was invented, a lot of research ideas and efforts were applied for

designing completely new and reliable medical devices. These developments helped to improve

quality of life and reduce expenses for treatment - especially for patients with chronic diseases and

those who are located remotely from the clinic, or with limited access to doctors. Wireless

connection provides benefits with respect to IMDs. It gives the opportunity to exchange data in

both ways: uplink and downlink. Most of the time data is transferred from IMD to the server,

which is located quite close to the person. Another benefit provided by wireless connection is the

immediate updating of electronic health records. On the other hand, there are some concerns - for

instance, cost, accuracy and security. On-body sensors, wearable devices and medical IoTs

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matched with smartphones have also become popular in connection with a variety of sport and

health applications.

In general, an implantable sensor is composed of two main parts, usually, the first part is attached

to the skin or placed inside of the body, and the other part is a receiver, located outside of the body.

The main purpose of the receiver part is to provide power to the IMD and deliver data to the doctor.

Some of the IMDs are independent, meaning that these devices can regulate their operations

depending on conditions. The main requirements for IMDs are based on power consumption, data

rates, biocompatibility, dimensions and security. Battery lifetime is a very significant issue, in

order to reduce chances for repeated surgery, it is vital to improving the lifetime of IMDs. Another

critical requirement for IMD are the dimensions of the device; the continuous technological

progress and the rapid development of semiconductors has led to rapid technological advances,

with the dimensions of the devices getting smaller. However, the battery occupies a significant

volume of the device, that’s why it is vital to move towards battery-less IMDs. Wireless powering

methods provide a wireless access to IMDs, such as RFID or NFC [125]. Human body resources

are able to produce energy such as thermal, movements, vibration etc, which can be converted into

electrical energy. Antennas are playing an important role in communications as well.

The thesis is organized as follows. Chapter 2 presents a short history overview of the implantable

biomedical sensors which gives the main sensor’s characteristics. Chapter 3 illustrates the main

digital modulation techniques which are employed in biomedical sensors and literature reviews

about recent publications. Chapter 4 focuses on the analysis of current harvesting methods for

implantable medical devices and the review of real devices which are based on various powering

methods. Chapter 5 is devoted to analysis of current challenges for implantable devices and

powering methods and how to mitigate them. Chapter 6 summarizes the conclusions obtained by

literature review and discusses future development perspectives.

1.1 Research Objective

The goal of the current thesis is to analyse and layout the advantages and drawbacks of wireless

powering methods for implantable biomedical sensors and to define the current and future

challenges of these approaches. This includes: analysis of the current situation on the market;

which methods are being used currently; what kind of cutting-edge technologies are employed for

implementing these approaches; and a review of modern publication about this topic.

1.2 Thesis Outline

This thesis will show: the analysis of existing power methods for implantable biomedical sensors;

the main characteristics of the sensors; a brief introduction to the history of implantable sensors; a

deep overview of modulation techniques which are being used currently for IMDs; research on the

advantages and disadvantages of each modulation method; and a review of recent publications

regarding these topics.

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2. SENSOR TECHNOLOGIES

2.1 History of implantable biomedical sensors

Nobody can deny that our future is closely related to our past. Science and medicine develop hand

in hand, with the medical device industry using cutting-edge technologies for the improvement of

the quality of life for any human being. In this chapter the history of implantable biomedical

devices will be explained.

People from all over the world suffer from different types of debilitating conditions such as

blindness, deafness, diabetes, heart failure and others. Before the 1920s, diagnostics of these

illnesses was a vital problem, which can reduce life expectancy or death. For instance, diabetes

became a significant problem, due to the great variety of people that suffered from it. Insulin was

one possible option for patients, but it demanded a lot of complex procedures. In the 1960s the

first insulin pump was designed. It was a great breakthrough in medicine, but unfortunately, the

dimensions of that design were enormous, and the mobility of the patients was therefore very low.

Later, advances in technologies of insulin pumps reduced the sizes of devices. In 1980, the first

insulin pump was developed with small dimensions; it can be compared to about the size of a

calculator. The first implanted insulin pump was introduced later, and it was a revolution for

medicine and diabetic patients. It brought a freedom to mobility and kept glucose level within

acceptable ranges.

Nowadays, modern devices are controlled via various software algorithms and are able to track

and adjust patients’ glucose levels wirelessly. Another advantage of modern devices is an

opportunity to check device status, the level of the battery, or possible malfunctions of the device.

The control is achieved wirelessly via a Bluеtooth interface into small tags that are placed on the

patient’s clothes.

The pacemaker device industry has followed the same path as the insulin pump; the first devices

were bulky and unreliable, and that’s why it didn’t have a positive impact on the community.

However, in 1930, Albert Hyman designed the artificial cardiac pacemaker. The dimensions of

that device were quite small for that time, but nowadays such devices have much smaller sizes

than 100 years ago. During the next 40 years, the pacemaker device has become smaller and

smaller.

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Figure 2. Cardiac stimulator before and after the wireless transfer

As a result, in 1960 the first successful implantation pacemaker was released, however, the first

devices suffered from a limited battery life. Researchers employed nuclear batteries as a power

harvester (plutonium), but due to the detrimental impact on health, they stopped using it. Later, in

1990, a new pacemaker was being released onto the market, employing a principle called “cardiac

resynchronization therapy”.

The next step in pacemaker evolution was the ability to connect devices wirelessly. The first

wireless pacemaker was introduced in 2009. The main advantages of the wirelessly powered

device are the ability to control and to make a configuration remotely, and the freedom of

movement for the patient. The use of wireless power transfer for pacemakers greatly reduced the

risk of health problems and surgery risks. A great variety of research has been undertaken in order

to optimize the transfer of wireless energy to pacemakers [136-138].

The evolution of pacemakers, insulin pumps, and mobile technology is presented in figure 3.

According to the picture, we can see comparisons between each device. It illustrates one significant

point, that the mobile devices industry is developing much faster the medical industry.

Figure 3. Timeline evolution of IMDs

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2.2 Sensor characteristics

The main purpose of any sensor is to react to external influences and to give instructions to the

system about these impacts. The simplest example of a sensor is a smartphone screen that reacts

to the touch of your fingers. This screen is equipped with a temperature sensor, allowing you to

react to the lightest and shortest touch of human fingers. Thus, sensors are able to convert an

impulse into a measured signal. Sensors are the vital part of the IMDs and are based upon a very

wide range of underlying physical principles of operation.

In this chapter the main definitions and characteristics of biomedical sensors will be presented, as

well as the importance of the measuring process. Nobody can deny that biomedical sensors should

be reliable, safe, and bio-compatible with a person’s body. In order to understand the main

characteristics of biomedical sensors, you need to understand the basic terminology which is used

in sensor design.

2.2.1 Sensitivity

Sensitivity describes the ratio between the input and the output signals. It shows how much the

output values change when the measured quantity changes. Some of the sensors measure very

small signals, for instance, sensors which measure electrophysiological signals have very sensitive

characteristics. Another example, some of the blood pressure transducers which have a sensitivity

rating of 10 mV/V/mm Hg; that is, there will be a 10-mV output voltage for each volt of excitation

potential and each mm of Hg of applied pressure. [161]

Figure 4. Sensors sensitivity [161]

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2.2.2 Measurement range

First of all, it is better to understand what “range” means. Basically, the range of any sensor is the

maximum and minimum values of an applied parameter that can be measured accurately. For

instance, some of the blood pressure sensors have the minimum limit of -50 mm Hg and the

maximum limit of +450 mmHg.

2.2.3 Precision

Precision refers to the “degree of a measurement’s repeatability in the same conditions”. If the

measurement results remain the same after many experiments, it means that the sensor has a high

precision.

2.2.4 Accuracy

Accuracy is “the expected error between the true values and actual values” measured by the sensor.

Accuracy can be expressed as a percentage of the full scale.

2.2.5 Linearity

The linearity of the sensor is the maximum deviation between the actual values of the

measurements and ideal results. Basically, linearity can be expressed in percent, according to the

equation (1)

𝐿𝑖𝑛𝑒𝑎𝑟𝑖𝑡𝑦 = ∆𝐿

𝑌𝐹𝑆∗ 100% (1)

where ∆𝐿 is the maximum input deviation, 𝑌𝐹𝑆 is the maximum full-scale input.

Figure 5. Linearity representation [161]

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2.2.6 Hysteresis

The hysteresis of the sensor refers to some sensors, in which input and output characteristics have

a non-linear trend, depending on input signal behaviour.

Figure 6. Hysteresis curve [161]

2.2.7 Resolution

The resolution of the sensor is the minimum detectable signal fluctuation. Resolution can be

expressed as a proportion of the full-scale readings.

2.2.8 Response Time

The output value of the sensors doesn’t change immediately when the input parameter change

occurs. The response time shows the time needed for a sensor to react when the input signal is

changed.

2.3 Frequency bands for wireless bio-medical implants

Currently, a great variety of wireless medical applications exist, such as implantable and telemetry

devices. One of the crucial requirements for wireless implantable sensors is power consumption

and small size. Most of the medical applications are used indoors and have quite a small range.

Wireless IMDs utilize different frequency bands and share the radio spectrum with other devices.

This leads to a lot of issues and concerns about performance, especially for applications which use

license-free spectrum - which can cause interference, fading, multipath propagation, and blocking

of the signals. A clear understanding of spectrum sharing (wireless coexistence and

interoperability) is significant for the design of medical devices [139].

Wireless coexistence defines the ability of a wireless system to work in a shared environment,

where other devices are using the same spectrum resources. There are three main factors which

define the wireless coexistence: time, space, and frequency. Coexistence is possible if any one of

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these requirements are met. First, sufficient distance between wireless networks. Second, the

frequency separation between wireless networks [127]. Third, low overall occupancy of the

wireless channel [2].

Table 1. Table of comparisons between different wireless method

Standard Frequency Range Data Range

Inductive coupling < 1 MHz < 1m Up to 30 kbps

Wireless Medical

Telemetry System

(WMTS)

608-614 MHz

1395-1400 MHz

1429-1432 MHz

35-70m >250 kbps

Medical Body Area

Networks (MBAN)

2483.5 MHz-2500

MHz

<1m Up to 1 Mbps

802.11a Wi-Fi 5 GHz 120m 54 Mbps

802.11g Wi-Fi 2.4 MHz 140m 54 Mbps

802.11n Wi-Fi 2.4 MHz-5MHz 250m 48 Mbps

802.15.1 Bluetooth

class 1

2.4 MHz 100m 3 Mbps

802.15.1 Bluetooth

class 2

2.4 MHz 10m 3 Mbps

802.15.4 ZigBee 2.4 GHz 75m 40-250 kbps

Medical Device

Radio

Communication

Service (MICS)

401-406 MHz 2-10m 250 kbps

Several frequency bands exist especially for medical devices, but many medical applications use

unlicensed spectrum called the ‘ISM band’, in other words industrial, scientific, and medical band.

Table 1 illustrates different wireless interfaces which are utilized by IMDs. These regulations and

standards must be respected by all manufacturers. Each frequency band is designed for certain

type of devices, for example, MBAN, WMTS, inductive coupling and MICS.

Inductive link is a reliable method for communication between an implantable device and a

controller which is located out of the person’s body. However, this method has some drawbacks:

the maximum separation between coils should not exceed 6 cm; the data rate is another issue - at

approximately 30 kbps, the speed of data transmission is very low; and interference is a significant

issue for inductive link medical applications, as the patient must be careful with these IMDs to

ensure communication between sensor and receiver is always possible.

Medical Implant Communication Systems (MICS) transmit data from implants inside the human

body to an external unit, in order to programme the IMD and control the patient’s data. An external

device or controller can communicate with other systems, such as the internet, for the remote

control of the patient’s conditions. The architecture of an MICS system is presented in Figure 7.

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The implantable sensor is able to receive commands and can be reprogrammed from a doctor

through a wireless connection. Patients can move freely with these sensors. Interference mitigation

is the key issue for MICS systems because all data s sent and received from IMDs should be

accurate and reliable. There are different error correction methods such as FEC and ARQ.

Figure 7. The architecture of MICS system

Wireless Medical Telemetry Systems are composed of sensors which measure important health

parameters, and transmitters for delivering data through a radio interface to the receiver. This

system utilizes different frequency bands from 600 to 1432 MHz all over the world. The UHF

band (608 MHz to 614MHz) was originally reserved for radio astronomy. Therefore, users using

this band should take into account that radio astronomy devices use the same band. The second

frequency band (1395-1400 MHz and 1429-1432 MHz) was originally designed for government

purposes such as military radar operations. Recently these bands have become available for

WMTS. However, some countries have not followed these regulations, therefore frequency bands

for WMTS need to be addressed and harmonised internationally. Without the acceptance of

frequency regulations all over the world, the safety and quality of service provided by wireless

medical devices are questionable. The WMTS is capable of two-way communication and

providing high data transmission rates. There are certain limitations in this system: restricted

bandwidth of only 14 MHz; video and audio transmissions are not possible; and each company

which designs WMTS equipment uses their own protocols and interfaces, therefore devices from

two different vendors are not able to communicate with each other.

Wi-Fi is the oldest wireless technology used in medical application, and has different sub-

standards such as 802.11a, b, g, n, ac, ad. Each of these methods have certain sections of the ISM

band. Wi-Fi allows transmission of both video and voice. Using the ISM band has advantages such

as a wide bandwidth of 83 MHz in total, and devices can communicate with other devices. Wi-Fi

networks can support a great variety of Wi-Fi compatible devices, such as access points, laptops,

and monitors. Frequency management is not required, great propagation characteristics, low power

consumption, easy to share spectrum resources with other devices. Wired Equivalent Privacy

(WEP) - an algorithm for ensuring the security of Wi-Fi networks is used to ensure confidentiality

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and the protection of transmitted data to prevent non-authorized wireless network users from

listening. There are two types of WEP: WEP-40 and WEP-104, differing only in the length of the

key. Currently, this technology is obsolete, as it can be hacked in just a few minutes. Nevertheless,

it continues to be widely used. For security in Wi-Fi networks, it is recommended to use WPA.

Wi-Fi Protected Access (WPA) is an improved version of the WEP protocol, therefore WPA has

better performance than WEP, however, this method still has some vulnerabilities. In order to

improve security characteristics, the next evolution of WPA has been released, called WPA2. This

method has better security characteristics for wireless networks. On the other hand, certain

limitations exist as well. The ISM band is unlicensed, and so other devices can cause interference.

Encryption Wi-Fi is relatively poorly protected against hacking. Range and Wi-Fi transmission

speed depend on the presence and intensity of the interference.

Wireless personal access network (WPAN) mainly uses two protocols: Bluetooth and ZigBee.

Bluetooth is considered as a very short-range wireless technology, with low power consumption

and small bandwidth. It is designed to replace cables and for communication between small

devices or sensors which are associated with the human’s body. Bluetooth utilizes the same ISM

band as Wi-Fi and can also transmit video and voice.

2.4 Security of wireless medical devices

Security of IMDs and medical information will be the significant problem in the next few decades.

Nowadays, most patients’ medical records are paperless, and devices are used for collecting health

data. Some of the devices are already available and very affordable, the main features being

monitoring of heart rate, daily activities, and sleep cycles, in addition to the ability to illustrate it

graphically.

Hospitals also widely use wireless technologies in diagnostics and treatment. The skyrocket

development of wireless technologies has a beneficial impact on patients who are receiving these

services. However, it is very important to keep this information secure and protect it from theft

and hackers. For example, if someone is trying to steal information from medical records, it could

be used for negative purposes or for the purposes of blackmail: therefore, it is important to store

this data in a secure manner. Security includes a lot of factors, such as confidentiality,

authentication, integrity, authorization, availability, and non-repudiation.

Recent studies have demonstrated the possibilities to attack IMDs. According to Halperin research

paper [3], various implantable devices such as cardiac or pacemaker devices, have a lot of

vulnerabilities to adversarial actions; this can result in information theft - such as personal

information and medical history - or in the influencing of heart rhythms. This is possible due to an

unprotected communication link between the IMD and the programmer. C. Li and A. Raghunathan

also demonstrated possible security risks on real examples of the systems which are currently

widely used, such as insulins pumps and glucose monitoring systems. The authors also offered

possible solutions to protect systems against these attacks. They proved that the wireless link

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between IMD and programmer should be controlled and secured to prevent illegal attacks from

intruders. [4-5]

There are many researchers from all over the world addressing the security problems of IMDs.

One of the common approaches is based on authenticating keys for establishing secure channel

communication between IMD and programmer. However, this approach is not compatible with

resource constrained IMDs. Hence, the IMD security should be based on tiny authentication

schemes and symmetric encryption [6-8] or employ a resource-rich personal device (e.g.

smartphone) to mediate communication between an IMD and an external programmer [9–11].

Another approach for the IMD security is associated with the accessibility issues of IMDs when

an emergency situation occurs. Suppose an unconscious patient with IMD enters an emergency

room (ER) of a non-primary-care hospital. In order for ER personnel to access the IMD the patient

has, some backdoors should be integrated for the programmer. Even though several techniques

[12–15] have been proposed, each of them have their own inherent security weaknesses.

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3. MODULATION TECHNIQUES FOR IMDs

Digital modulation - the process of converting digital symbols into signals compatible with the

characteristics of the communication channel. Each possible value of the transmitted symbols is

assigned to some of the parameters of the analogue carrier wave. Manipulation - the way digital

or pulse modulation, is when the carrier signal parameters change abruptly. When digital

modulation is used most often, it is a discrete sequence of binary symbols - binary codes. Encoded

primary analogue signal e (t), which is a sequence of code symbols

{EN} = EN (k) (n = O, 1, 2, 3, ... - a serial number of the character - the number of code positions;

m - code base, t. e. the number of its various components, which are converted into a sequence of

elements (chips) signal {Un (t)} of code symbols by exposure to high-frequency carrier wave UH

(t). As a rule, binary codes are used that m = 2. Usually, by modulating the frequency or phase of

the carrier in the radio the pulses vary as determined by a digital code. Digital modulation provides

much more information capacity and ensures compatibility with various digital data services. It

also increases the security of information, improves quality and speeds up access to the

communication system. On the other hand, the main drawbacks of the system with digital

modulation are a significant expansion of the occupied frequency channels of bandwidth, and the

need for accurate synchronization signals. Figure 8 illustrates the main types of digital modulation.

Figure 8. Modulation techniques used in biomedical sensors

The conventional wireless battery-less system link is used for 2-way communication between IMD

and other devices. Figure 9 depicts the architecture of a wireless battery-less interface. Due to

limitations in size and complexity of an implantable sensor, the signal processing part is located

in the external unit. Power required for the implant modules - including a central processing unit

(CPU), stimulators, and sensors - is transmitted by the external host via wireless interfacing. It

13

consists of two main parts: internal IMD and the external host system. The internal part is

composed of a power harvester, a modulator for sending the signals from the sensor and its status

for the external device, and a demodulator for receiving data from the sensor.

Figure 9. The architecture of the wireless battery-less interface

3.1 Principle of ASK modulation technique

Amplitude-Shift-Keying (ASK) modulation is one of the simplest digital modulation techniques

which is used for implantable biosensors. However, this method has several limitations such as

low data rates and high sensitivity for the amplitude noise. In ASK, the carrier amplitude is shifted

between low and high values depending on the data at the input of the modulator [8]. There are

two methods of ASK modulation which are perfectly suitable for the IMDs: coherent and non-

coherent schemes. The non-coherent scheme has some advantages such as low power

consumption, low complexity, and carrier phase detection is not used in this method. The coherent

method utilises carrier phase information for detection. The principle of ASK modulation is

illustrated in figure 10.

Figure 10. The principle of ASK modulation [209]

14

𝑆𝐴𝑆𝐾 = 𝑚(𝑡) ∗ 𝑐(𝑡) (1)

𝑐(𝑡) = 𝐴𝑐 ∗ cos(2𝜋 ∗ 𝑓𝑐 ∗ 𝑡) (2)

Where m(t) is the modulating signal (1 or 0), c(t) the carrier signal, and fc is the carrier frequency

It transmits 1s and 0s of data by transmitting with carrier or without a carrier. Bit 1 transmits with

a carrier frequency, bit 0 transmits without a carrier.

The modulation index, h, is the ratio between the maximum and minimum voltage levels of the

modulated signal.

ℎ =𝑚𝑜𝑑𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑎𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒

𝑐𝑎𝑟𝑟𝑖𝑒𝑟 𝑎𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒∗ 100% (4)

The modulation rate, r, is the ratio between data rate and operated carrier.

𝑟 =𝑑𝑎𝑡𝑎 𝑟𝑎𝑡𝑒

𝑜𝑝𝑒𝑟𝑎𝑡𝑒𝑑 𝑐𝑎𝑟𝑟𝑖𝑒𝑟∗ 100% (5)

3.1.1 ASK modulation method for implantable biomedical sensors

Reducing the chip area and power consumption are the key challenges for IMDs. In order to

mitigate for this, developers reduced the number of passive elements in these devices. The C-less

(no capacitor) ASK modulator for implantable neural interfacing chips has been developed by

[16]. It is employed to transfer external data and regulate a stable output power.

In 2003, Najafi and Yu designed low-power interface circuits for IMDs based on ASK modulators.

These circuits are composed of power-on-reset-block, power amplifier E class and low drop-out

regulator. In this method, the ASK modulator provides a bit rate of 60 kb/s. However, this design

has a very critical disadvantage with inaccurate synchronisation between clock and data signals

[17]. Later, Djemouai demonstrated a new CMOS ASK demodulator for IMDs. This design is

easy to implement, however, it has large dimensions. [18]. In order to solve that problem, Yu and

Baitullah implemented a novel low power ASK demodulator without DLL [19]. This design is

composed of low-power clock and data recovery IC. The main advantage of this method is accurate

synchronisation and reduced power consumption.

In 2007, Lee developed a C- and R-less low-frequency ASK demodulator for IMDs. As a result,

the number of elements was reduced to 12, with a total area of 0.003025mm2 and power

consumption at 1.01 mW [20-21]. A Low power consumption ASK modulator without passive

elements was introduced by Zhi Lui. They improved the data rate up to 2 Mbps and reduced power

consumption up to 84 μw, with a 0.3μm circuit size. A novel interface was represented by Li and

Zhang consisting of a digital processing part and an analogue front end [23]. The system interface

consists of the amplifier, low power circuit, ASK demodulator without passive components, and

15

a digital circuit. The power supply for this interface is 1.8V with power dissipation less than

2.75mW.

Another example of an ASK demodulator for implantable devices was implemented by G.

Gudnason; this modulator has been tested with fc in the range 1-15 MHz, with data rates up 100

kbit/s. Modulation indices for this case vary between 10-100% [24].

3.2 Principle of PSK modulation technique

Phase-Shift-Keying (PSK) is one of the most efficient digital modulations and is widely used in

different communication systems. In this scheme, the digital data is encoded in the phase property

of a carrier signal. Equation 6 presents corresponding 𝑠 signals for this modulation.

𝑆(𝑡) = 𝐼(𝑡) ∗ cos(𝜔𝑡) + 𝑄(𝑡) ∗ cos(𝜔𝑡 +𝜋

2) (6)

𝐴 = √2𝐸𝑠

𝑇𝑠 , 𝐸𝑠 = 2 𝐸𝑏 , 𝑇𝑠 = 2𝑇𝑏 (7)

where 𝐸𝑠 is the energy per symbol, 𝑇𝑠 is the symbol duration.

Figure 11. PSK modulation waveforms. [162]

In this system, a binary ‘0’ is represented by a signal packet, the phase of which coincides with

the phase of the previous packet sent, and a binary ‘1’ is represented by a signal packet with a

phase opposite to the phase of the previous packet. Such a scheme is called differential since the

phase shift is performed relative to the previously transmitted bit, and not relative to some

reference signal.

3.2.1 PSK modulation method for implantable biomedical sensors

Lower consumption and robust performance are the key advantages of PSK modulation. In 2004,

Hu and Sawan presented a demodulator with the use of COSTAS loop [25]. The BPSK

demodulator is composed of voltage-controlled oscillator (VCO), low-pass filter (LPF) and phase

16

shifters. The demodulator provides a high data rate of 1.12 Mbps with power consumption of 0.5

mW. The main feature of this design is a possibility to use BPSK and passive modulation, this

method allows for full-duplex data communication. However, this design remains complex.

Due to this issue, in 2008 Lu and Sawan released a new version of their design with two modulators

and demodulators based on OQPSK modulation [26]. This design has several advantages such as

high data rates, and low complexity - which results in this system having low power consumption.

Based on previous designs, Deng developed a version with modified COSTAS loop technology

and improved data rate transmission up to 8 Mbps, on 13.56 MHz carrier frequency and 0.75mW

power output [27].

Inductive coupling links are a very popular solution for transfer of power to the IMD, however,

this design is complex. In 2006, Zhou designed a new system which provides a high data rate

DPSK telemetry developed to mitigate interference without using a high-order filter [28]. This

system operates at 1MHz carrier frequency for power transmission and at 20MHz carrier

frequency for data transfer. This system has some disadvantages such as high-power consumption

and big size.

Later in 2008, Zhou developed a previous design using DPSK modulation to reduce the

interference for a dual-band configuration [29]. The data telemetry and power signals interfere

with each other and as a result, they produce interference. In order to solve this problem PLL less

scheme should be used. The demodulator provides up to 2Mb/s data rate, and this system operates

at 20MHz. For reducing interference at the receiver, DPSK modulation is used for IMDs through

dual-band telemetry [30]. The DPSK modulator provides up to 4 Mbps and operates at 22 MHz.

Some of the complex implants, such as retinal implants, require reliable and high-speed data

transmission. Elamary designed a BPSK modulator which provides high data rates up to 20 Mbps

and operates at 20 MHz frequency. The main feature of this design is an incredible data to carrier

frequency ratio of 100% and low power consumption. The BPSK modulator is implemented based

on the non-coherent method [31,32].

3.3 Principle of FSK modulation technique

Frequency – Shift- Keying (FSK) is another digital modulation method, which is widely used in

wireless transmission for biomedical applications. Figure 12 shows two oscillators forming s0(t)

and s1(t) oscillations at different frequencies. There is also an electronic key controlled by a digital

signal b(t) so that when a logical "1" is transmitted, a signal s0(t), is sent to the output and when a

logical "0" signal is transmitted, a signal s0(t) is sent. Thus, the frequency of the output signal is

"manipulated" depending on the bit sequence. BFSK modulation method can be described by

equations 8 and 9:

17

𝑆1(𝑡) = 𝐴 ∗ cos(2𝜋 ∗ 𝑓1 ∗ 𝑡) (8)

𝑆2(𝑡) = 𝐴 ∗ cos(2𝜋 ∗ 𝑓2 ∗ 𝑡) (9)

The main advantages of FSK are lower susceptibility to errors then ASK, simplicity in the

implementation, increase in the immunity of radio reception, power of the transmitter is better

used (since the power of the signal remains unchanged during the whole process of modulation),

and passband centralized between f0 and f1 with low Q to pass enough power for both frequencies.

On the other hand, there are some disadvantages: requires complex demodulator, large bandwidth,

and the synchronization between transmitter and receiver is complex.

Figure 12. FSK modulation principle. [163]

3.3.1 FSK modulation method for implantable biomedical sensors

FSK modulation is widely used in wireless transmission for wearable and implantable biomedical

sensors. It provides high data rates and low power consumption. In order to get low power

consumption, FSK transmitters should be adjustable in case of data rate, because different sensors

need to have various data rates, for example, transmitting physiological information requires a

transmitted data rate of about several kbps. However, for transmitting physiological image

information, a data rate of several megabits per second is required. Horng-Yuan Shih has

implemented this type of ultra-low FSK transmitter with power consumption ranges between 378

μW to 424 μW and data-rates varying from 200 kbps to 2 Mbps [33]. Zhinheng designed 2 and 4

FSK demodulators. This novel demodulator improves the bit error rate (BER) performance and

improves decision accuracy by generating additional zero-crossings - however, it has a very high-

power consumption at about 3mW from a 3V power supply.

In 2004 Ghovanloo and Najafi developed an FSK modulator based on the inductively coupled link

model [34]. This modulator consumes 0.38mW of power at 5V. Later, in 2006 Ahmet Tekin

18

designed a low power FSK modulator for transceivers in the MICS band. This modulator provides

up to 20 kbps data rate. [35] The size of the design is 0.18μm with CMOS process and a very low

power consumption of 33,41 μA at 1.5 V supply. Low power consumption and simple and reliable

architecture are significant factors for non-invasive implantable biomedical sensors. This

modulator developed by Zhu [36] has a relatively simple design; this circuit integrates the

modulation functionality into the oscillator itself by using the data signal to control the oscillation

frequency and producing carriers for different types of monitoring signals [35]. Supporting data

rates are from 450 kbps up to several Mbps, with low power consumption of 1μA at 2.5V.

Table 2. Examples of comparisons of modulation techniques for IMDs.

Modulation

technique

ASK FSK BPSK/DBPSK

Carrier frequency 1-250 MHz 2-433 MHz 10-20 MHz

Data rates 0.004-1 Mbps 0.18-1.5 0.18-0.5

Coherent or non-

coherent

Non-coherent Can be both BPSK is coherent,

while DBPSK can be

non-coherent

Power consumption 0.062-70 µW 33.41µW-3 mW 31.5µW-6.2mW

Noise performance Poor performance, as

it is heavily affected

by noise and

interference

Better noise

performance than

amplitude modulation

schemes

BPSK generally

shows better error

performance than

ASK and FSK, but

the error probabilities

are double with

DBPSK

Cost efficiency Low cost Simple, low cost

implementation

possible

Usually

implementation is

complex and more

costly that other

techniques

Applications Neural system

Physiological signal

Cochlear implant

Telemetry

Applications

Endoscope

ECG

General

Biological signal

Physiological sensors

Brain stimulator

Cardiac stimulator

Neuromuscular

stimulator

General

19

4. POWERING METHODS FOR IMPLANTABLE BIOMEDICAL SENSORS

Implantable biomedical sensors can be divided into two categories. The first category includes

sensors powered by sources surrounding the implants. Human daily activities such as motion,

breathing, sleeping, and body heat are great sources of thermal and kinetic energy. For instance,

everyday walking can produce 1500 mW.

Figure 13. Human’s power resources

This property can produce enough energy to generate a few hundred milliwatts. Kinetic energy is

an easily accessible source for IMDs. There are a great variety of possibilities for converting

kinetic energy into electrical energy. For example, piezoelectric materials, electrostatic, or

electromagnetic mechanisms are possible solutions for this purpose. The brief resumé of a

human’s potential power resources and various body actions are provided in figure 13.

20

Figure 14. Energy harvesting methods used in the biomedical sensors

Instead of using body resources, another feasible solution is to supply energy to the implantable

sensor through an ехtеrnаl unit. In this case IMDs can use ultrasonic, optical or electromagnetic

harvesting systems. Optical power delivering systems consist of a photovoltaic cell placed inside

of the IMD which receives power from a laser operating in the NF or IR range. Inductive power

transmission is one of the most reliable and efficient ways of delivering power to IMDs with small

sizes. The basic principle of this system consists of two antennas, one antenna is for TX and

another for RX, these antennas are used for power transfer. The ultrasonic method is one of the

more modern ways to transfer energy, due to the immunity from electromagnetic modulation and

high efficiency. Figure 15 depicts the efficiency of different powering methods for IMDs.

Figure 15. The efficiency of different powering methods for IMDs

21

4.1 Lithium batteries

The main purpose of the battery is to supply electrical energy to a portable device. There are two

types of batteries which are used in IMDs: primary and secondary batteries. Primary batteries

provide high output power and current and are used in pacemakers. Secondary batteries provide

relatively small output power and current, therefore these batteries are used in less power

demanding applications.

Some examples of primary batteries are: lithium-iodine, lithium-manganese dioxide, and lithium-

carbon monofluoride batteries. Greаtbаtch invented lithium-iodine batteries in 1973. Lithium-

iodine batteries have perfect parameters such as extended battery life and high voltage output,

which make them suitable for powering small electric devices.

These accumulators are used in pacemakers, cardiac defibrillators, and cochlear implants as a

reliable energy source. Li-ion batteries provide voltage up to 4V, showing better performance

than other types of batteries. These batteries have a very high-power density around 200 W·h/kg.

Lithium-manganese dioxide batteries have been developed for devices with additional features

which require power in the mW range. Lithium-manganese dioxide batteries were invented by

Ikeda in the 1970s; these batteries have outstanding characteristics including high energy density

and good storage and discharge characteristics. The main applications which employ these

batteries are pacemakers, neurostimulators, and drug delivery systems. The lithium-manganese

dioxide batteries are composed of a lithium anode which is placed in the centre of the cell and

surrounded by two cathodes. The battery has a high level of stability and low self-discharge rate.

The volume of these is 10.5 cm3 and it has energy density of 0.588 Wh/cm3, with a capacity of 2.5

Ah [210].

Lithium-ion batteries can be considered as secondary batteries. These batteries can be

distinguished by the type of cathode material used. The carrier of the charge in the lithium-ion

battery is the positively charged lithium ion, which forms a chemical bond with either graphite,

oxides, or metal salts, for instance, to form LiC6, oxides (LiMnO2) and salt (LiMnRON) metals.

As a conclusion, the primary systems employ lithium metal anodes and various cathode systems.

These batteries provide suitable power levels for different IMDs with different energy

consumption rates. Secondary batteries were developed for specific implantable devices which

have an option to be charged while remaining implanted.

4.2 Nuclear batteries

Nuclear batteries are another type of powering method for IMDs. The basic principle of nuclear

batteries is converting energy carried by particles emitted from radioisotopes into electrical energy.

Usually, nuclear batteries utilize plutonium, because it has a half-life of 88 years and output power

reduced by 10% in 10 years. Nuclear batteries were introduced in the medical industry in 1973.

22

Figure 16. Nuclear pacemaker [164]

The main advantages of these batteries are that they have a very long life, with stable output

characteristics. For instance, the Betacel produced by Medtronics, Inc can provide 50 μW of power

with small dimensions. [38] On the other hand, there are some disadvantages such as the extreme

toxicity of plutonium. [39-40]; only 1g can have a fatal effect.

Nuclear batteries are mainly used in pacemakers with each pacemaker having special shielding to

prevent additional radiation contamination. As a result, plutonium powered pacemakers produce

radiation at approximately 100 mrem per year. Normally a person gets an average 360 mrem per

year from different sources such as solar radiation and medical sources. Consequently, plutonium-

based pacemakers have been recommended even for a pregnant woman.

Nuclear-powered pacemakers stopped being produced in 1980, due to the introduction of lithium

batteries.

4.3 Piezoelectricity power generators

The piezoelectric effect was revealed by the brothers Curie in 1880 [40]. The Curies proved that

special structures are able to provide a proportional electrical polarization with respect to applied

mechanical stress [211]. Energy conversion through piezoelectricity is illustrated in figure 17.

Figure 17. Energy conversion scheme

23

𝑚�̈� + (𝑏𝑒 + 𝑏𝑚) ∗ �̇� + 𝑘𝑧 = −𝑚�̈� (10)

Where y is the base displacement, k the spring constant, z is the output tip displacement, m the

lumped mass, bm and be are the mechanical damping and electrical damping coefficients

respectively. The power output of the system, P, is calculated using:

𝑃 =𝑚𝜉𝑒𝐴

2

4𝑤(𝜉𝑒+𝜉𝑚) (11)

Where 𝑤 operation frequency, A is the acceleration input of the input vibration,

There are two types of piezoelectric effect: ‘direct’ and ‘opposite’. The ‘direct effect’ is used to

transform mechanical energy into electrical energy and the ‘opposite effect’ has the same principle,

but vice-versa. Piezoelectric materials have three operational modes: transverse mode refers to

when the force applied along a Y-axis generates charges along the X-axis; the Longitudinal effect

and the Shear effect produce charges proportional to the applied forces.

Figure 18. The Basic work principle of piezoelectric IMD

Piezoelectric sensors convert mechanical energy - for example, body motion - into electrical

energy. Physical and chemical processes inside of the human body can be employed in order to

provide a power source for various wearable devices and IMDs. There are two types of body

motion: continuous and discontinuous. Continuous motions such as breathing and heart beating

can generate up to 2W. Another type of motion is discontinuous, for instance, walking, jogging,

and cycling.

There are four different materials which can be used for piezoelectric devices: ceramics (lead-

zirconate-titanate (PZT)), polymers, single crystals, and composites. Composites, crystals and

ceramics have better characteristics than polymers. However, polymers are more suitable for use

cases where the IMD will be subjected to a large amount of bending. Most of the piezoelectric

devices are built on ceramic elements - especially PZT - due to cheap prices and outstanding

characteristics. The efficiency of piezoelectric material depends on resonant frequency; in order

24

to get maximum power output, piezoelectric devices should precisely tune to the resonant

frequency. The piezoelectricity coefficient depends on materials.

There are two main conventional configurations of piezoelectric devices: cantilever beams and

disks. Disks are divided into two categories: cymbals and diaphragms. The cantilever shape is one

of the most common configurations for converting vibrations into electrical energy, due to its

simple design and low resonant frequency. Figure 19 depicts two types of cantilever shape, A

figure shows unimorph and B figure show bimorph design.

Figure 19. Configurations of piezoelectric cantilevers [170]

The unimorph design is composed of thin piezoelectric and non-piezoelectric layers. In such a

configuration, there is only one active layer (piezoelectric) and the other layer is just a steel plate.

Another configuration is bimorph, composed of two piezoelectric layers, hence both are active.

This configuration is used for improved power output of the design. In piezoelectric layers poled

directions are usually orthogonal to the planar directions. After literature review about this topic,

a great number of energy harvesters are designed with the use of bimorph or unimorph

configuration. However, the bimorph structure appears more beneficial and widely used, due to

better output power characteristics for the same volume of device compared with the unimorph

design.

Cymbal piezoelectric converters are another type composed of a steel end-cap and a piezoelectric

disk. The working principle is as follows: when the axial stress is applied to the steel surface of

the device, this causes deformation which converts and amplifies the axial stress into radial stress

in the piezoelectric disc. The energy efficiency of this system is higher than for the cantilever

beam-based systems. As an example, cymbal piezoelectric devices can produce up to 52 mW under

70N force, with dimensions of 29 mm in diameter with 1mm thickness [207]. However, this design

is not suitable for applications with high magnitude vibration sources.

Figure 20. Configuration of cymbal piezoelectric device [170]

Stress

25

The circular piezoelectric diaphragm is another type of design. It operates in the same way as

piezoelectric cantilevers. To construct a piezoelectric circular diaphragm transducer, a thin circular

piezoelectric ceramic disc is first bonded to a metal shim and then the whole structure is clamped

on the edge, while piezoelectric cantilevers are only clamped at one end of the cantilever beam

[170].

A great variety of researchers have investigated how to use discontinuous motions as a source of

energy [41-43]. Obviously, continuous motions such as breathing provide less power compared

with discontinuous motions. For instance, heartbeat vibrations are a rich energy source, meaning

it is possible to convert vibrations into energy supply for pacemakers with the use of linear and

non-linear PEH [182,183]. Researchers from the MIT Lab designed a device which generates

electrical energy from kinetic energy (human walking). This device converted mechanical energy

into electrical energy for wearable applications. At the same time, Starner [174] proposed his idea

about the implementation of piezoelectric elements which can be integrated inside shoes and rotary

generators that are able to collect and store energy from running shoes. In 2001, Paradiso designed

integrated piezoelectric elements called polyvinylidene fluoride. These piezo elements are placed

in shoes and generate electricity from bending of the foot [44]. One element is located in the heel

and another in the toe region; the design provides up to 8.3 mW. However, this design is suitable

only for persons who are able to move freely. Another energy efficiency design was proposed by

Rome [175]; he designed a spring-loaded rucksack to gather electrical energy from walking.

However, that design was based on an electromagnetic system. Grandstrom decided to replace that

system with piezoelectric straps [176]. Zhu developed a piezoelectric generator to convert the

movement of a knee joint into electrical energy [179]. Renaud proposed a piezoelectric harvester

to collect energy from human limbs [177]. A head-mounted piezoelectric harvester was developed

by Voix and Delnavaz, which is able to collect energy from jaw movements. Later, Ertuk

developed an advanced broadband harvesting system based on a piezomagnetoelastic structure,

which is more efficient than other existing systems [178]. This is a short review of designs based

on PEH, which are able to generate energy from the human walk. Moreover, different PEHs have

been developed in order to collect energy from multi-directional vibrations [180-181] but are not

applicable to walking harvesting, due to the nature of the movements. However, in 2017 Fan and

Lui [172]

Figure 21. A prototype of shoe mounted PEH [44]

developed a shoe mounted non-linear PEH which was able to convert energy from various motions

produced by the foot. The design is composed of a piezoelectric cantilever beam, a crossbeam, and

26

a ferromagnetic ball [172]. The output power of PEH is up to 0.35 mW, however, it works only in

certain scenarios - specifically when the walking velocity is within a certain range. Nevertheless,

this PEH is able to produce a large amount of power compared with other PEHs, but on the other

hand the output of most current PEHs provide slightly lower output power in comparison with

electromagnetic harvesters; For example, some electromagnetic harvesters produce up to 0.5W in

similar scenarios.

As a conclusion, piezoelectric power generators are quite efficient, but on the other hand, they

need a lot of movement to generate sufficient power.

4.4 Thermoelectricity

Presently, chemical power suppliers are a very popular solution for low-powered devices. Despite

continuous quality improvements of chemical power sources, they still have some limitations such

as short battery life, large geometrical dimensions, and negative environmental impact.

Consequently, researchers from all over the world are looking for better power sources.

Thermoelectric generators are one of the possible approaches, because they have long battery life

and a high level of reliability. Thermoelectric harvester technologies are based on the Seeback

effect. The German physicist Thomas Johann Seebeck discovered that a temperature difference

between two different electrical semiconductors produces a voltage difference between the two

substances. When heat is applied to one of the conductors, heated electrons move towards the

cooler ones. The voltage output produced by the Seebeck effect is only a few microvolts.

On the other hand, the Seebeck effect generates low voltage output - typically a few microvolts -

only if the temperature difference is big enough. The efficiency of these generators is possible to

calculate using the Carnot principle:

𝜂 = 1 − 𝑇𝑐

𝑇ℎ=

𝑇ℎ−𝑇𝑐

𝑇ℎ (12)

Where 𝜂 is the Carnot efficiency, Th and Tc are the hot and cold temperatures in Kelvin. The

efficiency of thermoelectric generators is dependent on the resistance coupling. The physical

model of a thermoelectric generator is illustrated in figure 23. The temperature differences between

different parts of the body form a temperature gradient. This gradient can be applied to the

thermoelectric module, which produces sufficient electric power. There are two modes of

thermoelectric generator (TEG): maximum power (or matched load) mode, and mismatched load.

Let us consider one case in which the required output power must be 1.5 V, thus thermoelectric

power force should be equal to 3V. According to experiment measurements, if the thermal gradient

between the temperature of a person’s body and the ambient temperature equals 5 C the Seebeck

coefficient equals 200 mkW/K, the thermoelectric power force 3 V, and the number of branches

in TEG equal 3000.

27

In matched load mode, the resistance of TEG, r, is equal to load resistance, R, at the maximum

current value, Imax. Let’s assume that maximum efficiency of TEG could be reached with the

following values: Imax = 36 mkA and R =41,7 kOm, the number of branches, N = 3000, where the

height of each branch equal 5mm with cross-sectional dimensions 0.06×0.06 mm and the spacing

between each branch is 0.02 mm. Thus, the total geometric dimension of TEG is 4.4x4.4x5mm.

However, cross-sectional dimensions as described above have some drawbacks such as complex

technological implementation. On the other hand, increasing cross-sectional dimensions causes

mismatched loads between power supplier and load, and can also decrease the efficiency of TEG.

Therefore, it is very important to define the best branch dimensions which can provide high

efficiency of the TEG. For these purposes, we can use the mismatched load mode.

Mismatched load mode

The maximum efficiency defined using the next equation (13)

𝜂 =1

4

𝑇1−𝑇2

𝑇1𝑍

𝑇1−𝑇2

2 (13)

Where T1 and T2 are the temperatures on different sides of TEG, Z quality factor of material.

According to calculations, if the temperature difference is 5C, the maximum efficiency that can

be reached is 0.4%. Let’s investigate the dependency between efficiency and cross-sectional

dimensions of the branches. In this case, efficiency calculates as equation 14

𝜂 =𝑊

𝑄ℎ (14)

𝑄ℎ = 𝑘𝑆𝑚

𝑙∆𝑇𝑁 (15)

Where 𝑄ℎ is thermal power, which perceives TEG, k is the coefficient of thermal conductivity,

and Sm is the cross- section square of the branch. From those equations we can observe that

increasing cross-section dimension of the branches causes a boost in thermal power and it

decreases the efficiency of the TEG.

Figure 22. Dependence of the efficiency of the linear branch cross-section dimensions [158]

28

From figure 22 we can see that increasing linear cross-section dimensions from 0.06 to 0.5mm,

the efficiency of the TEG reduces by 5-10 times. However, we can improve efficiency by

decreasing cross-section dimensions of TEG. Thermoelectric generators are usually made from

bismuth telluride or polycrystalline silicon-germanium film.

Figure 23. The architecture of thermoelectric generator

The thermoelectric module consists of two types semiconductors, p and n. The voltage generated

by this module can be calculated according to the following equation:

𝑉 = ∫ (𝑆𝐵(𝑇) − 𝑆𝐴(𝑇))𝑑𝑇𝑇𝑐

𝑇ℎ (16)

Where SB and SA are the Seebeck coefficients of the two materials.

There are a great variety of existing systems which are implemented based on thermal energy

conversion. João Paulo Carmo [158] presented one possible solution for powering low-power

electronics. Table 3 illustrates power consumption rates for different IMDs.

Table 3. Power consumption rates for different IMDs

Implanted Device Power Requirements

Cardiac Pacemaker 30-100µW

Cardiac Defibrillator 30-100µW

Drug Pump 100µW-2mW

Cochlear Implant Up to 10mW

29

As a conclusion, we can observe that thermoelectric generators are only suitable for low power

IMDs, because thermoelectric generators are able to produce only a few microwatts. In order to

increase power output, thermoelements should be connected in a cascade. However, this can cause

problems such as the big size of the system, and low reliability.

4.5 Electrostatic generators

The working principle of electrostatic generators is based on electrostatic induction. These

generators convert mechanical energy into electrical energy by moving part of the transducer

versus an electrical field [212].

Figure 24. Conceptual view of the electrostatic generator

The conventional electrostatic generator is composed of two conductive plates that are electrically

isolated through a capacitor or air. The distance between two plates changes due to the human’s

body movement. There are two methods of converting mechanical energy into electrical: with

fixed potential or with a fixed charge. [46] The first type is described in figure 24 whereby

movement of the plate generates a current through the capacitor - often it is called an ‘electret-

free’ type. The second method is with a fixed charge; when an external force is applied to the

structure, it causes a change in the voltage across the capacitor - sometimes this is called an

‘electret-based’ type. Usually, electrostatic generators are made from silicon using cutting-edge

manufacturing technologies such as micromachining fabrication. This provides suitable

integration capabilities with electric circuits.

The first type of electrostatic converters is electret-free, this is a passive structure that is able to

transform mechanical energy into electrical energy. Charge-constrained and voltage-constrained

are the conventional methods of energy conversion.

30

Figure 25. Energy conversion principles of electret-free electrostatic IMDs. [208]

• Charge-constrained method

The charge-constrained method is relatively easy to implement on a real design. The conversion

cycle begins with the external injection of the charge, and due to polarization effect, the whole

structure reaches maximum capacitance Cmax. At this moment capacitor C has a charge Qcst under

the voltage Umin. Then, the circuit becomes open and the capacitance of the systems moves to the

minimum level. However, the capacitor keeps charge Qcst at a constant level, causing an increasing

voltage across the capacitor C. When the capacitor reaches Cmin, electric charges are moving from

the structure and feed it to the load. The total amount of energy calculated is shown in equation 17

Eq =1

2𝑄𝑐𝑠𝑡

2 (1

𝐶𝑚𝑖𝑛−

1

𝐶𝑚𝑎𝑥) (17)

• Voltage-constrained method

The voltage-constrained method has the same principle at the beginning of the process, with the

structure reaching maximum capacitance Cmax and polarized at voltage Vcst and Cmax. The main

difference, in this case, that the voltage is then kept at a constant level with decreasing capacitance,

causing an increase of the charge of the capacitor; this generates a current that is scavenged and

stored. The next step is charge transfer to the load. The total amount of energy is calculated in

equation 18

EV = Vcst2 (Cmax − Cmin) (18)

The second type of electrostatic converters is the electret-based converter shown in Figure 26. This

type is quite similar to electret-free structures; however, electret-based converters don’t need any

external energy for polarization and have a direct energy output from the deformation of the

structure.

31

The electret-based harvester is composed of two plates with fixed and movable electrodes.

According to Gauss’s law, the electret induces charges on electrodes and counter-electrodes. The

electric charge on the electret can be calculated as:

𝑄𝑒 = 𝑄1 + 𝑄2 (19)

When the structure is subjected to a mechanical stress, one of the plates moves away from the

electret, changing the air gap and then the electret's influence on the counter-electrode, leading to

a reorganization of charges between the electrode and the counter-electrode through load R [208].

Figure 26. Electret-based electrostatics conversion model.

All of the electrostatic generators are based on capacitors; there are various types of capacitor

structures used, such as in-plane gap closing converter (a), in-plane overlap converter (b), in-plane

converter with a variable surface (d) and out-of-plane gap closing converters (c) (Figure 27).

Figure 27. Capacitor structure

Tashiro designed an electrostatic generator which is able to provide up to 50 μW when placed in

motion by a force simulating the cardiac signal. [47] Later, in 2002 he further developed this

design, and his research group tested this electrostatic generator (ESG) with animals, obtaining a

32

heartbeat up to 190 bpm. [48] In 2006, Miao proposed a resonant less MEMS ESG for the IMDs,

which produced approximately 80 μW. [49] A great variety of ESGs are available in the present

time on the market [50]. Meninger [217], demonstrated the design of an electric circuit which can

convert external vibrations into electricity with the use of a variable capacitor, which is applicable

for low power applications. The theoretical results showed a device with relatively small

dimensions - 1.5cm x 1.5cm – which can generate 8.6 mW from an excitation of 500 nm at 2.5

kHz. Furthermore, the energy output can be improved by adding an additional capacitor connected

in parallel. In 2013, Deterre proposed an energy conversion method based on heartbeat vibrations.

The working principle of the design is that the package of the implant is deformable, thus blood

pressure effects on the electrostatic element convert vibrations into electrical power. Simulation

results showed that a 25-layer electrostatic element with 6mm diameter is able to collect up to

20µJ per heartbeat.

As a conclusion, the main drawback of this type of transducers is an additional source of energy

required to operate, and usually the amount of energy which the generator produces is much

smaller. On the other hand, due to dependence on the motion force, an active pre-charge system

gives the opportunity to dynamically optimize the generator for the applied motion.

4.6 Electromagnetic harvesters

The principle of energy conversion is based on the electromagnetic induction when an

electromotive force (EMF) is induced in a conductor moving in a magnetic field and crossing its

magnetic lines of force.

𝑉 = −𝑑𝜙

𝑑𝑡 (20)

Where 𝜙 is flux linkage and V is induced EMF. Usually the generator’s design is composed of

coils. Therefore, the output voltage or EMF calculated as

𝑉 = −𝑑Φ

𝑑𝑡= −𝑁

𝑑𝜙

𝑑𝑡 (21)

Where Ф is the total flux linkage, N is the number of turns. Ф calculated as

Ф = ∑ ∫ 𝐵 ∗ 𝑑𝐴

𝐴𝑖

𝑁𝑖=1 (22)

Where B is the magnetic field flux density over the ith coil’s turn.

Consequently, such a conductor can be regarded by us as a source of electrical energy. There are

two types of mechanical generators that generate electrical energy. The first method is based on

relative motion where the generating system is fixed, and the second method is a rigid body motion

and uses the inertia force of a weight on the generator [51].

33

Figure 28. Types of mechanical generators: a) relative movement, b) rigid body

Typical movements of the human body can be used as a possible solution for powering biomedical

sensors based on electromagnetic generators. For instance, a small electromagnetic generator can

produce 400 μW from human walking [52]. As another example, the heartbeat can produce up to

200 μW. [53] High-performance bulk magnets, multi-turn and macroscale coils are readily

available; nevertheless, the main challenge for the MEMS fabrication technology utilized in this

approach is the poor properties of planar magnets. [54]

As a conclusion, electromagnetic generators are usually less efficient and more bulky then

piezoelectric generators. [124] A comparison of the three techniques in relation to different

characteristics is presented in Table 4.

Table 4. Comparison of Electromagnetic, Electrostatic and Piezoelectric powering methods.

Features Electromagnetic

harvesters

Electrostatic

harvesters

Piezoelectric

harvesters

Energy density Low A decrease in the

separation between

the capacitor plates

increases the energy

density

High

Output voltage Up to 0.1V 2-10V 2-10V

Frequency

dependence

No dependence No dependence Highly frequency

dependence

External source No required Required in electret-

free converters

Not required

MEMS

implementation

Difficult to

implement in MEMS

Easy to implement in

MEMS

Difficult to

implement in MEMS

Drawbacks Low efficiency in

small sizes and low

frequencies, ohmic

loss in coils

Electrets can be used

to provide the initial

charge needed,

parasitic capacitance

losses

Materials can be

expensive, poor

coupling of piezo

films at micro scale

34

4.7 Ultrasonic energy transmission

This method is one of the possible solutions of transferring energy to the implantable biomedical

devices which receives a high level of attention recently [55-58]. What sets apart ultrasonic

transmission from other methods is its high efficiency, safety for the human body, and stability

against interference from other devices. [59] Typical ultrasonic power systems operate on

frequencies above 20 kHz and need a propagation medium in order to deliver energy from TX to

RX. For IMDs, ultrasound systems transfer energy through tissues to the implant, where it is

converted to electrical energy through the piezoelectric transducer. The conventional design of an

ultrasonic energy transmission system is shown in figure 29. The typical applications for

ultrasound energy system are cochlear implants, pacemakers and neurostimulators. [146-148,150].

The system is composed of transmitter, power-system and receiver. According to the equation 23,

it defines the received power, Pr:

𝑃𝑟 = 𝜂𝐴𝐸𝐼𝐴𝐶𝑆А (23)

Where 𝜂𝐴𝐸 is electric efficiency pf the receiver, 𝑃𝑟received power

The transmitter transforms electrical energy into mechanical energy and then acoustic energy and

is composed of an ultrasonic oscillator electrically excited to generate mechanical vibrations on

the surface of the TX; these vibrations cause acoustic pressure waves. The receiver has the same

functions but in reverse order. [57] Based on the Huygens-Fresnel principle, each point on the

transducer can be treated as an independent source of radiation, and the acoustic field pattern can

be found at the vector sum of all the point-radiating sources [144]. The pressure field P at an

observation point L (x, y, z) is given by the Rayleigh integral in equation 24:

𝑃(𝑥, 𝑦, 𝑧: 𝑡) =𝑗𝑘𝑝0𝑐0𝑢0

2𝜋𝑒𝑖𝜔𝑡 ∫

𝑒−𝑗𝑘𝑅

𝑅𝑑𝑆

𝑆 (24)

Where λ is the wavelength, ω angular frequency, k wave number, R is the distance between point

source and the observation point, u0 is the vibration velocity amplitude, c0 phase velocity of the

wave, p0 density of the medium [165].

35

Figure 29. Schematic of ultrasonic IMD. [165]

Power transfer efficiency depends on different aspects, for instance, losses in the transducer,

tissues and rectifier losses, impedance matching. Losses have a negative impact on the system

performance.

Using piezoelectric components made from lead zirconate titanate have a more beneficial effect

on energy efficiency conversion than piezoelectric polyvinylidene fluoride, due to different

electromechanical and mechanical factors k and Q.

It is very important to choose the right operating frequency because it affects tissue attenuation,

transducer thickness, the distance between the natural locus and the sizes of reactive elements in

transmitter and receiver sides [165]. In order to provide sufficient quality of energy transfer, the

whole system should work close to its resonance frequency. The main challenges for ultrasound

transfer systems are addressed below:

The human body is complex and diverse in terms of organs and systems, each of them has different

densities and acoustic impedance. For example, acoustic impedance is quite high in bones,

consequently all ultrasound waves will be reflected.

Another problem is related with implants which are placed under the skin, where the spatial

separation can reach several acoustic wavelengths, therefore it effects on power transfer efficiency

(PTE). Another changes which affect on PTE are changes in tissues and ambient temperature

[153]. Overall it leads to serial limitations on the battery life and system performance.

The first commercial ultrasound energy transmission device was developed in 1988, with an

operational voltage of 10V and 2.25MHz frequency. This device generated an output power of

1.5mW/cm2. Ultrasonic systems are primarily used for medical imaging, for instance,

ultrasonography. The typical frequency range for this application is between 3-6 MHz. In

comparison, acoustic waves have lower speed than radio waves for given frequencies, therefore

acoustic waves have smaller wavelengths. As an example, for the given frequency range, the usual

36

acoustic velocities in human tissues range from 1500-2000 m/s, the wavelengths 0.4 – 0.8 mm.

Another possible design is described in [145].

In 2009, Tower proposed a design which is suitable for monitoring, it converts the energy of a

surface-applied ultrasound beam to a high-frequency current [60]. Later, Zhu [61] used ultrasonic

waves to harvest energy for the IMDs with 21.4 nW. In [128], a device was presented with an

experimental Mbit/s ultrasonic transmission through ultrasonic phantoms, while in [129], [130]

ultrasonic wireless transfer to power mm-sized implantable devices was demonstrated. The

possible applications based on ultrasound powered method are described in [133-134]. Utilizing

the ultrasound method for powering IMDs can push devices towards miniaturisation, due to short

wavelength (1.5mm at 1MHz), and allows for greater energy efficiency [135].

The main advantage of this method is operational wavelength because long wavelengths are able

to penetrate deeper into the human’s body, moreover, this method is safe and effective. On the

other hand, there are some drawbacks in this system, such as expensive equipment.

4.8 Photovoltaic infrared power radiation

Infrared energy transmission is another power delivery method for IMDs, such as pacemakers or

brain implants. The main component of the system is a photovoltaic cell. Initially, photovoltaic

cells were developed as a renewable energy source, however, after years of intensive research,

these devices can be adopted as a power source for IMDs. Photovoltaic cells used in aerospace

applications are able to transform up to 50% of light energy into electric power [201]. The principle

of this energy conversion is based on the photovoltaic effect, where two different materials in close

contact can produce an electrical voltage when the materials interact with light.

Figure 30. Infrared power radiation system [202]

Conventional infrared power radiation systems are composed of external light sources - for

instance, lasers located on top of the skin emit light through biological tissue layers, and the light

is received and transformed into electric power [214]. According to various research papers human

tissues have quite a high optical transmittance coefficient in near-infrared light, hence it is possible

to implement wireless powering systems for IMDs based on this infrared power radiation

principle. Usually, these systems are tuned to a wavelength of about 900 nm [203]. Infrared

wavelengths are able to penetrate much deeper in tissues than other wavelengths of light, but they

37

still suffer from various negative factors such as scattering and absorption caused by tissues [204].

Consequently, biomedical implants based on this principle typically use NIR wavelengths. In 2007

Sond and Simeral demonstrated a real design of a photovoltaic pulse neurostimulator. This system

is composed of an infrared laser tuned at 852 nm wavelength and using optical fibre [205, 206].

4.9 Inductive coupling

There is another technique for delivering power wirelessly to implantable biosensors, called

inductive coupling, which was first used in an artificial heart. This method is based on Faraday’s

law. In 1831 Faraday discovered that power could transfer wirelessly based on the principle of

magnetic induction [140]. Later, power transmission over a large distance without any cables was

achieved by Nicola Tesla [62]. Tesla’s work was based on resonance and it was a groundbreaking

achievement in that period. During the first part of 20th-century, invention and achievements were

slowed down; however, due to military research, new types of high-frequency oscillators, which

improve the techniques of wireless power transmission, have been developed.

Figure 31. The architecture of Inductive coupling powering method

The NF resonant inductive coupling method is one of the most reliable wireless power transfer

methods. This method has a lot of evidence that it is robust and safe for use in medical applications,

as an example, it has approved results by the Food and Drug Administration (FDA).

In this method, the source part is placed under the skin surface and the receiver is placed outside.

The overall design of inductive coupling scheme is illustrated by figure 31. When a voltage is

applied to the primary coil, L1, it excites magnetic flux. This magnetic flux creates an EMF in the

coil, L2, due to electromagnetic induction. The highest voltage is achieved when both source and

receiver are tuned to the same resonant frequency, f. This frequency can be calculated by equation

25 [63-64].

𝑓 = 1

2𝜋√𝐿𝐶 (25)

Where L is the magnetic inductance, and C is capacitance

38

The transmitting and receiving coils are poorly coupled due to spatial separation. The induced

EMF can be defined as

𝜀 = ∮ �⃗�

𝜕Σd𝑙 = −

𝑑

𝑑𝑡∫ �⃗� d𝐴

Σ (26)

Where �⃗� is the electric field; �⃗� magnetic flux density; d𝑙 is the vector element of the contour 𝜕Σ,

d𝐴 is the area vector element.

Mutual inductance is one more parameter which has a significant role in the design, as it defines

the mutual inductance between two coils L1 and L2. The coupling coefficient can be calculated as

[65]

𝐾 =𝑀

√𝐿1𝐿2 (27)

The efficiency of the inductive link is defined as a ratio between the power delivered to the load

and the power supplied to the primary coil and is called the power transfer efficiency:

𝜂 =𝑘2𝑄1𝑄2

(1+1

𝑄2+𝑘2𝑄1)(𝛼+

1

𝑄2) (28)

𝜂 =𝑘2𝑄1𝛼

(1+1

𝑄2+𝑘2𝑄1)(𝛼+

1

𝑄2) (29)

Where Q1 and Q2 are quality factors for the coils, k the coupling coefficient, 𝛼 the coefficient

(equal to wC2RL), C2 the capacitance of the second coil, w the frequency, and RL the resistance of

the load. Equation 28 and 29 represent link efficiency for parallel and series resonant circuits

respectively. The quality factor defines the efficiency of the inductive link.

The wireless power efficiency depends on distance, frequency and matching between L1 and L2

coils. Normally, these systems operate at a frequency of 20 MHz. There are four different schemes

for how passive systems can be connected, such as series to series, parallel to parallel, series to

parallel, parallel to series. These topologies are depicted on figure 32. These topologies have very

poor performance under weak coupling conditions, but the series connection has better PTE than

the parallel topology in strong coupling mode [141]. Both topologies provide the same amount of

power to implants, however, serial topology achieves this by using high current and low voltage.

On the other hand, parallel topology achieves the same result, but with high voltage and small

current. [142-143] According to electromagnetic theory, rectifiers have better operational

characteristics at high voltage and low current, therefore the parallel topology is more widely used

for IMDs.

39

The number of coil turns is another important parameter. It depends on coil shape and wire

properties, such as material or line size. The diameter of coils is an important parameter which

affects the performance of the inductive link; increasing the diameter of the coils increases the link

efficiency. In IMDs, size is a vital parameter, thus the diameter of implantable coils should be

minimized, but the diameter of the external receiver coils can be larger to increase link efficiency.

It is also desirable that coils are flexible, in order to be safe for the patient. [151-152].

The next important parameter is the number of turns; if we increase the number of turns in the

coils, the performance of the system will be better. Another important factor is the spacing between

primary and secondary coils. Therefore, the relative position of IMD and receiver play an

important role. Furthermore, if the patient is moving, it can cause misalignment and interrupt the

connection between transmitter and receiver.

Figure 32. Types of topologies

There are many challenges in designing the optimal inductive coupling link. First of all, the IMD

consumes different current at various time periods, therefore producing different loads on the

scheme. However, the scheme is usually designed only for a certain load. Therefore, the link can’t

operate efficiently all the time. Secondly, IMDs are housed inside the human body, which moves

a lot during the day, causing misalignments and artefacts becoming inherent. The inductive power

link performs poorly under misalignments; in order to improve performance, new advanced

designs are needed. Thirdly, TX and RX use high-quality factor coils which decrease the PTE

value [156-157].

An inductive power source able to produce approximately 50mW was presented by Catrysse in

2004 [67]. This system operates at 700 kHz. Later, Ghovanloo designed a system based on SOC

which can produce around 50 mW and operates at 5 MHz frequency. [66]

40

4.10 Far Field Communications

This type of energy transfer was also developed for powering IMDs [184]. This scheme employs

the same type of antennas but tuned to different resonant frequencies than those utilized in

inductive coupling. Far-field communication has certain advantages in comparison with inductive

coupling, especially from a practical point of view. For example, these antennas can deliver energy

to many IMDs at the same time and it is not necessary to adjust antennas accurately. As a result,

the far field communication method can provide power to multiple implants located reasonably

far from TX antennas; this is therefore highly suitable for implantable sensors which are located

deep inside tissues. There is another advantage of using this method; it works perfectly with small

antennas tuned to high-frequencies, up to several GHz. On the other hand, this configuration has

a few disadvantages, for instance low power transmission efficiency in comparison with inductive

coupling. This can be explained due to large attenuation at high frequencies and decreasing

radiative power density at the far-field zone. Also, this technology can cause negative

consequences for the human body, especially at high frequencies, due to RF absorption in tissues

[185,186]. Consequently, the Federal Communication Commission (FCC) and FDA defined strict

rules regarding maximum output power [187-189]. There are few examples of real applications

which are based on the far-field communication principle: glaucoma monitoring devices [190],

neural signal recording devices [191-192], blood monitoring systems [193], and IMDs in the

ocular system [194].

4.11 Mid-Field Communication

Mid-field communication defines a range between far-field and near-field communication. This

type of energy transfer was discovered in order to overcome disadvantages of far and near field

communications [195-197]. Applications based on mid-field communication are able to provide

various vital features, for instance recording, monitoring, and stimulating neural activities. A few

years previously, a research group from Stanford University developed a mid-field wireless power

transfer system which has a maximum transmission efficiency greater than far-field and near-field

communication. The key component of this design is a 2 x 2 slot antenna array. By applying

different RF waves in different phases on 4 s ports of the antenna, optimal current density is

formed, and the combination of inductive and radiative modes are generated inside the tissue [166].

Due to the deep propagation of RF waves inside the human body, focusing effects take place,

causing the formation of small focal points. Hence, the mid-field communication method has one

significant advantage; with use of this method, RF waves can propagate further, and with higher

transfer efficiency, than inductive coupling [198]. Another example of an IMD based on mid-field

communication is a miniature pacemaker. When placed deep inside of the body with a 2 mm x 3.5

mm size coil, the power output is about 2 mW [199, 200].

As a conclusion, mid-field power transfer gives an opportunity to design an electronic parts at the

millimetre scale and the device can be placed nearly at any location of the body. Due to the rapid

development of the semiconductors industry, these devices are able to monitor and process data

equal to, or even faster than, their battery-powered counterparts. With the use of advanced

41

technologies such as MEMS, logic units will allow interactions with the tissue environment to be

engineered. Table 5 illustrates the advantages and disadvantages of different electromagnetic

power methods.

Table 5. Comparisons between different electromagnetic power methods

Type Inductive coupling Far-Field

Communication

Mid-Field

Communication

Advantages High transfer efficiency

Low tissue heating

Long transmission

distance

Multiple unit powering

Miniature powering

Mid-field

transmission

Miniature

powering

Disadvantages Short range transmission

Large coil size

Sensitive to alignment

Low transfer efficiency

Tissue heating

Tissue heating

Sensitive to

alignment

4.12 Bio-fuel cells

Bio-fuel cells cаn prоduce electricаl energy thrоugh cоnversiоn оf biоmedicаl energy, invоlving

chemicаl reаctiоns such аs оxidаtiоn аnd reductiоn. Biо-fuel cells utilize nоble metаl cаtаlysts tо

prоduce electrоns frоm fuel оxidаtiоn. An externаl circuit then conveys electrоns tо a cаthоde

where electrоns оxidize аnd produce energy. Figure 33 illustrates this process.

Figure 33. The design of bio-fuel IMD

Currently, bio-fuel cells based on glucose are widely used in various applications. The basic

principle of these cells relies on the electrochemical reactions between oxygen and glucose.

Glucose can provide up to 16 kW per gram, generating 12 electrons per molecule during the

oxidation process. Relying on the metal catalyst used, bio-cells can be divided into two types: the

first type is the Microbial Fuel Cell (MFC) and the second is the Enzymatic Bio-fuel Cell (EBC).

42

The MFC uses some microorganisms, therefore, the oxidation of the molecule of glucose cedes

electrons to the bacteria. When this event occurs, the bacteria metabolism is inducted into a Redox

cycle. The products of reactions in this method are water and carbon dioxide, and these

components are fully compatible with the human body. However, if the device with MFC is

damaged inside of the person’s body, it can cause contamination of the human body by bacterial

cells. The efficiency of MFC is time-dependent; according to research papers, the efficiency of

MFC decreased significantly after two hours of usage with maximum values 0.0249 mA and 0.326

V.

The EBC is another type of BFC that uses enzymes as a catalyst to oxidize its fuel rather than

precious metals. Enzymes are proteins that are used in chemical reactions as an intermediate link

in the person’s body. The two main components which participate in the enzymes reactions are

substrates and products. The speed of reactions depends on the enzymes and substrates. Molecules

of the enzyme are very specific, so the reaction starts only when a link between substrates and

enzymes is established. The power efficiency of the BFC depends on which kinds of enzymes are

in use and the size of the system. According to publications, a power density up to 1.39 mW cm-2

can be achieved.

The concept of bio-fuel cells (BFC) was discovered in 1911 by MC Potter, when he noticed that a

culture of the bacterium E. coli can generate current up to 0.5mA. Later, due to development of

MFC technology, a new idea of energy generation from treated biological waste has been offered.

In 2003, Mаno [218] published his study about a bio-fuel cell which is able to generate 2.4 μW of

power and 0.52 V.

This technology has great advantages, such as environmental friendly effects, and high efficiency

and bio compatibility with the human body. Additionally, the technology has the ability to operate

at temperatures between 20 – 40C. On the other hand, there are some drawbacks still remaining.

First of all, it is challenging to maintain biocatalysts over a long period of time. Secondly, the

output power level is usually around few microwatts, giving limitations in some applications.

43

5. CURRENT CHALLENGES AND FUTURE PROSPECTIVE OF IMDs

The biomedical device manufacturing industry is one of the fastest developing industries in the

world. eHealth care applications have a great opportunity for continuous monitoring. There is a

great variety of medical applications which are vital for human beings; they can help to diagnose

and monitor health, these include pacemakers, EMG, ECG, and brain implants. Most of these

devices are battery operated. Therefore, energy management is a key issue, especially for longer

durations. There are some important limitations for implantable sensors: patient safety, reliability,

low power consumption, small dimensions, high data rate, and low cost. There are various factors

that lead to the limitation of wireless energy distribution. First of all, it is not often possible to

design and implement transmitters and receivers in a small enough size to make it suitable for a

miniaturized implantable system. The second issue is the range of the energy transfer. According

to the latest publications, the current systems are able to transmit energy only in a relatively small

range, typically at a maximum of one meter, causing problems with real implementation. The third

problem is power efficiency; usually this varies between 45% and 80% in comparison to battery

or wired-based technologies. Recent studies show short summaries about the advantages,

drawbacks, and power generation of different powering methods for implantable biosensors.

5.1 Power management

The wireless communication link consumes a lot of energy from the wireless implantable system.

The main goal is to reduce power consumption of the RF transmitter in order to extend the lifetime

of the implantable part. The FCC allocated the frequency range between 402-405 MHz for medical

implant communication services (MICS), and frequency ranges between 608-614MHz, 1395-

1400MHz and 1427-1432MHz for medical telemetry [215].

Batteries are the main barrier in the design of IMDs. Batteries can be classified into two types:

single-use non-rechargeable and rechargeable. For instance, pacemakers and defibrillators have

one-time use batteries. Insulin pumps and pacemakers have rechargeable batteries. The single-use

batteries must be replaced when the output power is lower than the minimum level for device

operation. That level can be monitored by wireless inductive telemetry. Single-use batteries must

be changed by surgery, while rechargeable batteries can be recharged wirelessly. Telemetry

systems consist of two main parts: a receiving coil, which is located inside of the body, and a

transmitting power system.

The telemetric link can be used for 2-way communication or full duplex communication to transfer

data such as saved health information about the patient and information about the external unit;

this link also gives an opportunity for programming devices or communication between several

44

IMDs utilizing the same wireless link. There are various modulation techniques used for

communication between IMDs and power stations. For example, AM, FM, PM, FSK, and PSK

are also used for data communication; a detailed description of each of these modulation

techniques can be found in chapter 3. Data transfer requirements of the IMD is the key factor for

selecting modulation method. The lower operational frequencies provide low data rates and higher

frequencies provide high data rates. The second factor for modulation method selection is the

availability of power or bandwidth. It is very important to choose the correct modulation technique

because it increases channel capacity, reduces power consumption of the transmitter, improves the

quality of the signal, and enables reliable transfer of data in the presence of disturbances such as

noise, deep fading, and interference [212].

There are different energy harvesting methods that have been invented, such as physical, chemical,

or mechanical energy generation. This allows the IMD to get energy from natural body motion or

the physiological environment wherein the IMD has been placed. Sontag proposed to use high

density electroactive polymer brushes of poly-thiophene, made by means of a surface-initiated

Kumada-type polycondensation reaction, to power IMDs [84]. Rapoport presented a novel design

of a bio-fuel cell that produces up to 3.4 μW, which utilizes glucose oxidation. [85]

5.2 Biocompatibility

Biocompatibility of implantable devices is a vital issue. When the IMD is placed inside of the

body, biological tissues react to the foreign object. Consequently, in addition to traditional invasive

surgical procedures, special conditions should be also be fulfilled in order to eliminate infections

before implantation, which can cause lethal consequences. For example, the probability of

infection during implant installation ranges from 1%–17%. A great variety of sterilization methods

exist at the present time, such as dry heat sterilisation, gas plasma sterilisation, gamma radiation

sterilisation etc. To avoid blood and tissue incompatibility due to implants, these devices should

be packaged within biocompatible materials, as discussed by Park [86]. Most IMDs, such as

pacemakers, cochlear implants and implantable cardioverter defibrillators, are sealed by a

biocompatible material. Currently, the most effective biocompatible materials are titanium and its

alloys, noble metals and their alloys, biograde stainless steels, some cobalt-based alloys, alumina,

zirconia, quartz, tantalum, niobium, titanium-niobium alloys, fused silica, silicon, and some

biocompatible polymers [87-97].

Biofouling is a significant issue which influences how IMDs work. In fact, implantable devices

will be exposed continuously to different elements of the body. After some time, bio-organisms or

biomolecules within the body will interact with the surface of the IMD, reducing the functionality

of the device [98,99]. In order to prevent biofouling, several methods of сoating have been

invented; they are added to the device package. This, however, causes an increase in the size of

the device. Voskerician and Schmehl demonstrated that different types of coatings such as gold,

silicon nitride, silicon dioxide, silicon carbide are able to significantly reduce biofouling.

45

5.3 Health issues related to wireless power transfer energy

Health risks should be taken into account at the designing stage of wireless power transfer system

for IMDs. Energy radiation is able to warm up biological tissue due to the thermal effect; if this

effect has a very high level, it can have a negative impact for the patient’s health. In order to

prevent harmful effects and regulate the usage of frequency bands, the American National

Standard Institute standard dictates the electromagnetic field strength limits for frequency ranges

between 300 kHz and 100 GHz. The IEEE C95.1-1991 standard regulates the electric and magnetic

field strength limits for public use for the frequency range between 3 kHz-300 GHz. [100]

Table 6. IEEE C95.1-1991 Maximum electric and magnetic field strength limits for public use

Frequency

Range

(MHz)

Electric

Field

Strength, E

(V/m)

Magnetic

Field

Strength, H

(A/m)

Power Density, S (mW/cm2) Averaging

Time

|E|2,|H|2

(minutes)

E-field H-field

0.003-0.1 614 163 100 1E6 6

0.1-3 614 16.3/f 100 6

3-30 1842/f 16.3/f 900/f2 6

30-100 61.4 16.3/f 1 6

100-300 61.4 0.163 1

f/300

10

10

6

300-3K - - 6

3K-15K - - 6

15K-300K - - 616000/f1.2

SAR (specific absorption rate) is an important factor which indicates the amount of RF exposure

by a human. SAR is used to measure the amount of energy absorbed by a body which is exposed

to an electromagnetic field. It is defined as the power absorbed per mass of the tissue, with units

of W/ kg or mW/g [216].

𝑆𝐴𝑅 = ∫𝜎(𝑟)|𝐸(𝑟)|2

𝜌(𝑟)𝑑𝑟 (30)

Where 𝜎 is the electrical conductivity of the sample, E is the RMS electric field and 𝜌 is the sample

density.

The normal amount of RF radiation for a human is approximately 90 MHz; therefore, standards

are very strict in this case. The SAR value should have fulfilled standard requirements [101-102].

The average SAR level for the human body should be within 0.5 W/kg, as this is the main

requirement according to the IEEE C95.1-199 standard [103]. The maximum SAR level permitted

from cellular radiation is 1.7 W/Kg [104].

46

5.4 Size of the IMDs

The size of IMDs must be as small as possible. The battery usually occupies a significant

proportion of the sensor. For example, smartphones are getting smaller with every new generation,

whilst medical vendors are constantly striving to produce compact and user-friendly devices. The

main issues with sensor batteries are their limited lifetime and the possibility of a negative effect

on human health. This is why vendors now try to produce battery-less sensors and provide wireless

power transfer solutions. The lithium battery is able to provide 1400-3600J/cc, and allows sensors

to operate for a long period of time. Methanol based fuel cells can be more efficient than lithium

batteries because they can provide up to 17.6kJ/cc. Wirelessly powered IMDs in mm- or sub-mm-

sized form factors have the potential to enable advances in applications like neuromodulation,

localized biosensing, and targeted drug delivery [131].

The antenna is embedded and has an impact on most implantable devices, meaning antenna design

is a significant issue - due to limitations on the size, material and shape of the antenna. The

propagation environment changes according to different factors, including patients’ age, weight

gain or loss, and posture changes [106]. Only biocompatible, high-quality materials can be used

for example, platinum or titanium, which have better performance in comparison with basic copper

antennas. The dimensions of an implant depend on application and location, bringing limitations

in the design. For instance, the loop antenna is perfectly suitable for biomedical application,

especially for implant communication applications, due to lack of dependency of permittivity. On

the other hand, there are some problems if the IMD has a metal case or shield.

5.5 Frequency band selection

The carrier frequency is another vital factor in a wireless power system. There are a great variety

of IMDs working on frequencies less than 1 MHz, with different applications using various

frequencies, for example, RFID uses 125-135 kHz. The main reasons why vendors should use a

specific ISM band are as follows: to reduce interference between other devices which utilize the

same frequency band, to provide safety for human health, to improve the robustness of the

communication link, and to achieve a higher throughput on the network. Deep fading occurs when

a received signal is distorted due to various obstacles or interference from other devices. This can

cause the inability of the wireless node to maintain uninterrupted communication with other nodes,

and as a result it reduces the reliability and energy efficiency of the system. Interference control

and mitigation methods for deep fading are especially significant for medical applications, where

uninterrupted communication is required. The interference problem - which can appear from in-

body, on-body and off-body communication links - can be solved by using different types of

diversity, for example ‘channel’ or ‘antenna’ diversity. This issue becomes challenging if the

signals are able to attenuate, or become distorted, in or around a human body; the effects depend

on the body size, volume of muscles, and any body movement.

47

5.6 Antennas

The antenna is an important part of the implantable sensor, and for its radio link communication.

Antennas for medical applications should meet certain requirements such as radiation efficiency,

operating bandwidth, patient safety, and size. Body wireless communications systems (BWCS) is

a very prospective type of future communications. BWSC are composed of three types of

communication: on-body, in-body, and off-body. The first type defines communication between

various wearable devices. The second type means communication with external networks. The

third type can be defined as communication to an IMD or sensor.

Figure 34. Types of body wireless communication

Recent research publications for implantable antenna design:

5.6.1 Dual-band operation antenna

The main principle of dual-band antennas, is a switching between two modes - wake-up, and sleep

- thus saving energy and extending the lifetime of the battery. Usually, dual-band implantable

systems operate in the sleeping mode in the ISM band and change to the MICS band only in wake-

up mode. In 2008, Karacolak [69] designed a miniature DBIA which operates in ISM and MICS

bands for continuous glucose-monitoring applications. Later, in 2010, Sanchez-Fernandez [70]

introduced a novel dual-band micro-strip patch antenna for two bands MICS (402-405 MHz) and

ISM (2.4-2.48 GHz). In 2014, Zhu Duan [71] designed a new differentially dual band antenna

which operates at MICS and ISM bands; the antennas are made from a parylene-c material, with

a tiny flexible case of 179.0 mm3 and 186.3 mm3 respectively. Characteristics of the

communication system between an external half-wavelength dipole and implanted antennas at two

resonant frequencies presented in [126]. Another different approach can be found in [72-75]. In

2017 H. Behvar proposed novel dual port planar antenna for inductively coupled IMDs. This

antenna utilizes two circularly polarized implantable antennas.

It is well known that wireless communications suffer from multipath propagation, due

to various obstacles, movement of people, weather conditions, and other effects. For these cases,

an implantable circularly polarized antenna is desired. Real-life examples of these can be found in

48

[76-78]. The circularly polarized patch antenna was designed for 2.4-2.48 GHz band, with

10×10×1.27 mm3 dimensions. The simulated and measured impedance bandwidths in cubic skin

phantom are 7.7% and 10.2%. Another example of a circularly polarized antenna design was

published in 2015 by Li-Jie Xu [79]. That circularly polarized antenna operates in the 902-928

MHz ISM band. Right and left-hand polarization can be implemented, depending on the positions

of feed and shorts. The dimensions of that antenna are 13x13x1.27 mm. The simulated results

show that a wide bandwidth of 18.2% can be realized with |S11| below -10 dB and an axial ratio

below 3 dB. The simulated realized gain is -32 dBi at 915 MHz.

5.6.2 Implantable antennas for wireless power transfer

There are two types of batteries used to power IMDs: single-use and rechargeable. The main

drawback for the single-use battery is that of its replacement, usually requiring surgery, for

example pacemaker batteries. However, wireless power transfer can help to recharge batteries for

different IMDs without surgery. Implantable antennas were designed for wireless data telemetry

and play a key role in near/middle/far-field power transfer. Various types of implantable antennas

were studied for wireless transfer in [81,82]. As an example, in 2011 Fu-Jhuan Huang [81]

implemented an implantable antenna that perfectly operates at three bands, including MICS and

ISM bands, by utilizing an antenna with a specific shape. That radiator has a very small size of

10x10x10x2.54 mm.

5.6.3 Integrated implantable antennas

One of the key requirements for an implantable antenna is small size, especially in the case where

the implant should be placed in the head or eyes. With the advent of СMOS technology, this

problem has been solved. CMOS technology helps to reduce the antenna size and achieve high

integration of the whole implantable system. The first separate transmission and reception using

on-chip antennas in eye environment for intraocular pressure application was designed by L.

Marnat in 2012 [82]. These small antennas fit on a 1.4-mm 3 CMOS (0.18 μm) chip in addition to

the rest of the circuitry.

5.7 Security

Secure communication is one of the most critical issues for various IMDs, and consequently a

great variety of studies have already been presented in the literature on this topic. Some of the

security challenges conflict with basic IMD requirements. Firstly, on battery life; for IMDs the

duration of battery life should last for many years inside a patient’s body, however, security

algorithms consume an additional amount of energy, thus degrading battery life. The second issue

is that of adaptability. In order to provide high-level security for various IMDs, adaptable methods

that do not require any modification of IMDs are significant. Thirdly, availability; the main

purpose of authentication algorithms is to prevent of unauthorized access to the IMD. On the other

hand, in the case of emergency situations, when the patient is unable to turn off authentication

system, the IMD should be available to a doctor for a treatment, even if he or she were not

49

previously authorized. Another challenge in IMD security is reliability. Security mechanisms

introduce additional complexity into the system, increasing the susceptibility of IMDs to software

bugs and hardware impairments that lead to malfunctions. Therefore, security algorithms should

be robust and provide high-level reliability.

Most of the security techniques in wireless communication based on cryptography are used for

impersonation and eavesdropping [113]. However, most of the common security approaches may

not be suitable for IMDs. For instance, secret key storage and data encryption are memory

demanding operations [112] which might be a problem for the proper deployment of IMDs,

because they have limited memory. For example, in [110] and [11], authentication is implemented

based on a friendly jamming algorithm. When the Guardian detects a spoofing attack by

adversaries attempting to hide their existence from Guardian and gain access to the IMD, Guardian

activates a defensive jamming mechanism to notify the IMD about the threat. Note that, this

protocol requires a collaboration with the IMD and Guardian. Moreover, encryption with stored

and pre-shared keys conflicts with accessibility requirements of IMDs, for example in the

emergency case when a doctor should be able to have access in order to treat patients, even if he

or she were not previously authorized. One of the possible solutions to overcome these challenges

was introduced by [114], suggesting the use of psychological signals. In practice, this is used for

electro-cardiogram (ECG) and photoplethysmogram (PPG) signals in [115]. Another possible

solution is where inter-pulse intervals (IPIs) of heartbeats are exploited to generate security keys

in [116]. The ECG based security key generation has a lot of advantages in comparison with other

methods in the literature [110], because of higher randomness as compared to other physiological

signals, such as blood pressure, heartbeat, and temperature. The randomness of physiological

signals (PVs) is still an open question for practical scenarios [117].

For security communication control between the IMD and the programmer, and considering the

accessibility requirements, Denning discussed the usage of wearable external devices [119] and

introduced Cloakers [118] as a new direction in IMD security. Another popular trend for

preventing spoofing attacks is anomaly detection-based authentication; this method observes the

IMD related activities, either in the body or between transceivers, to understand the legitimacy of

incoming commands. Anomaly detection-based methods in [120,121] use the changing patterns

of different parameters [122], such as time of arrival and received signal strength (RSS).

All of the security methods mentioned above should be evaluated according to the most significant

threats. In eavesdropping resistant methods, PV based methods have some important advantages

in comparison with common cryptography methods, such as accessibility and availability, but they

suffer from low randomness and noisy measurements. However, in [123], researchers cope with

these problems, but still have disadvantages against intelligent attackers, as presented in [124]

which contributes modifications in IMD algorithms. A friendly jamming-based method in [111]

provides protection for IMDs without the issues mentioned above. In addition, this method is more

power efficient due to all security activities undertaken by an external device. PV-based key

generation may be more convenient for authentication, but less effective for secrecy against

50

eavesdroppers as it is a real-time operation and does not exhibit high entropy. However, there are

also other issues relating to authentication which should be considered.

Table 7. Current trends addressing various threats to IMD security

Methods Eavesdropping Impersonation Jamming

Cryptography

(Regular or PV-

based)

+ + -

Anomaly detection - + -

External device

deployment

+ + -

FHSS and DSSS + + +

Maintaining security against different types of attacks for communication with IMDs is still an

open problem, with researchers globally trying to find effective solutions.

5.8 Amplifiers for IMDs

The power amplifier is one of the main components in an implantable device, and it defines the

efficiency of the system. There are currently several different types of amplifiers in existence and

used in present applications. The ‘A class’ is the best class, with low signal distortion and high

linearity. The ‘B’ and ‘C’ classes operate with the same principles and properties but with lesser

quality. The most popular classes of amplifiers for IMDs are ‘E’ and ‘F’. The ‘E’ class amplifier

requires only one active device, and as a result has a shifting resonance frequency and a decrease

in the output power. On the other hand, ‘F’ class usually utilizes two devices at the same time,

causing an increase in power consumption and an increase in the dimensions of the system.

Table 8. Comparisons between different power approaches

Energy powering

methods

Advantages Drawbacks Generated power

Thermoelectricity Long life-time Low output power 5.8µW

1µW

180µW/cm2

Piezoelectricity High output voltages

High capacitance

Expensive

components

Coupling coefficient

linked to material

properties

1W

0.33µW

2.1-69.8W

Electromagnetic High output currents

Long life

Low output voltages

Expensive

components

Low efficiency

40-200µW

1.1mW

400µW

Electrostatic High output voltages

Low cost materials

Low capacities 36µW

58µW

51

No direct mechanical

to electrical

conversion

80µW

Bio-fuel cells Environmentally

friendly

Biocompatibility with

human body

Short life-time

Low output power

2.4µW

Ultrasonic Data transfer

May be used for

different depths

Low output power

Side effects

1.5mW/cm2

Inductive coupling High data rates

No batteries needed

Limited carrier

frequency due to

tissue absorptions

Side effect

19mW

150mW

50mW

6.15mW

Lithium batteries Compatible with

flexible electronics

Size, Toxicity 210 W*h/kg

5.9 Future prospects

With the fast development of cutting-edge technologies, the medical industry has changed a lot in

many aspects. There has been a transformation in the traditional way of diagnosis and monitoring

of various diseases and their prevention in the early stages. Moreover, the roles of doctors and

nurses have changed as patients are now able to monitor their health status with cutting edge

technologies within e-care, IoT, and mobile health. These technologies provide continuous

diagnosis and can transmit data to doctors who can be located a hundred kilometres away.

According to figure 35, depicting a future scenario, power is needed not only for powering the

IMD, but also for maintaining the wireless communication link; as a result, the lifetime of the

battery is reduced.

Figure 35. Overview of the future system

52

According to this scenario, an implantable device is powered by an external device, for instance,

a wеаrаblе bеlt. Simple, robust and affordable implantable sensors hold great promise for various

types of medical procedures. The design of small wireless sensors, such that they fit easily into an

implant or sensors, is an important factor for the real-time measurement of various health

parameters. For instance, future research may include utilizing multiple sensors in a multi-axial

load cell to monitor in-vivo loading in the spine in real time.

Energy efficient hardware should always be employed in IMDs; existing wireless technologies

have a high current peak and mostly use duty cycling between standby and active modes in order

to reduce the average current drawn. Cutting-edge technologies in the hardware for sensing

technologies can minimize the peak current drawn [106]. Thus, devices can work on low low-peak

currents. These cutting-edge technologies are wake-up radios [107] and low-power listening

devices [108], which are able to reduce power consumption by standby listening. Crystal is one of

the costly, heavy, and power demanding components of the sensor, therefore inventing the crystal-

less radio [109] is one possibility to reduce dimensions, price, and power consumption of the IMD.

Fоr wirеlеss trаnsmissiоn, IMDs cаn transmit information tо a smаrtphоnе. Different types of

algorithms can be implemented for filtering and sorting information for doctors or storage in

healthcare centres. Fast data processing and storage space are the main requirements for future

IMD systems; cloud-based systems are feasible solutions for these criteria. Future systems will

help to improve the quality of life for patients, save time for diagnostics, reduce expenses, and

increase the efficiency of treatment. In thе nеxt fеw dеcаdеs, miniаturizаtiоn оf IMDs is expected,

bеcаusе this аllоws clоsе proximity tо thе оrgаn and the ability to trеаt the target directly. Wirеlеss

pоwеrеd implаnts will hаvе a dеcisivе аdvаntаgе, duе tо their lack оf cоnnеcting lеаds - thе

wеаkеst pаrt оf а bаsic implаnt.

53

6. CONCLUSIONS

Life becomes more pleasant with good health. The invention of IMDs has extremely improved a

patient’s quality of life. This work summarises the various harvesting methods which can be used

for powering wireless IMDs; their characteristics, current challenges, and future vision. On the

one hand, there are different battery-based methods (battery, thermoelectricity, electrostatic

methods), on the other hand, there exist wireless power transmission methods (magnetic coupling,

ultrasound methods). The goal of the research has been successfully achieved. The main

challenges for wireless implants are biocompatibility, security and lifetime of the batteries.

However, in order to solve problems with the delivery of power to the IMD, various approaches

have been developed to replace battery-based systems for powering implants. The independent

system method employs the body’s energy, for instance, everyday human activity such as walking,

jogging or breathing, body temperature to produce electrical energy for harvesting for the IMD.

These methods are based on electromagnetic, thermoelectric, and electrostatic principles,

however, these methods suffer from various challenges, such as low output power and limited

implant location choices. Systems with an external unit - operating based on inductive coupling,

optical charging, or the ultrasonic transmission - provide great possibilities for transferring data

and energy simultaneously. All these methods have advantages and disadvantages. Hybrid systems

such as [154-155] can improve the quality of energy transfer. According to research, energy

transfer by wireless resonant magnetic coupling is the most highly regarded technique for

supplying implants. Nevertheless, this method still has limitations. For a secure wireless power

transfer (WPT), the skin should not be exposed to an energy density higher than 10mW/cm2,

which is established by ANSI. Resonant coupling WPT is a method that can be technically

improved in order to satisfy an implants’ energetic needs with respect for the international limits

of magnetic field exposure to the human body. Moreover, different modulation techniques, their

principles, and their characteristics have been reviewed. FSK modulation is the most suitable

approach for digital transmission, for instance, whilst PSK modulation is now widely used for

implantable wireless devices, employing a coupling inductive link to transmit both data and power.

The main challenges in wireless power and data transmission are data rate, power consumption,

modulation technique, and operating frequency. Consequently, choosing the most suitable

modulation technique is very important to achieve targeted data rates and power consumption of

the system. In order to fulfill the aforementioned requirements, advanced technologies should be

applied; for instance, artificial intelligence can be used to provide sufficient data rates and a stable

RF signal, offering great opportunities for flexibility in updating the data to the IMD, and in

automatic frequency control applications.

54

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